Bluetooth: L2CAP: Start servicing configuration request from remote

To make channel operational in basic mode a process of BR/EDR CoC
channel configuration needs to be performed in both directions. Remote
peer is mandated as well to inform local about its segmentation
requirements. Here is added such request handler focused on grabbing
remote MTU which is mandatory in channel setup.

> ACL Data RX: Handle 12 flags 0x02 dlen 12  [hci1] 12.133539
      L2CAP: Connection Request (0x02) ident 3 len 4
        PSM: 1 (0x0001)
        Source CID: 64
< HCI Command: Host Number of Completed Packets (0x03|0x0035) plen 5  [hci1] 12.133631
        Num handles: 1
        Handle: 12
        Count: 1
> HCI Event: Number of Completed Packets (0x13) plen 5  [hci1] 12.133792
        Num handles: 1
        Handle: 12
        Count: 2
< ACL Data TX: Handle 12 flags 0x00 dlen 16  [hci1] 12.141644
      L2CAP: Connection Response (0x03) ident 3 len 8
        Destination CID: 64
        Source CID: 64
        Result: Connection successful (0x0000)
        Status: No further information available (0x0000)
< HCI Command: Host Number of Completed Packets (0x03|0x0035) plen 5  [hci1] 12.142054
        Num handles: 1
        Handle: 12
        Count: 1
< ACL Data TX: Handle 12 flags 0x00 dlen 16  [hci1] 12.143276
      L2CAP: Configure Request (0x04) ident 6 len 8
        Destination CID: 64
        Flags: 0x0000
        Option: Maximum Transmission Unit (0x01) [mandatory]
          MTU: 48
> HCI Event: Number of Completed Packets (0x13) plen 5  [hci1] 12.150945
        Num handles: 1
        Handle: 12
        Count: 2
> ACL Data RX: Handle 12 flags 0x02 dlen 16  [hci1] 12.150959
      L2CAP: Configure Request (0x04) ident 4 len 8
        Destination CID: 64
        Flags: 0x0000
        Option: Maximum Transmission Unit (0x01) [mandatory]
          MTU: 128
> ACL Data RX: Handle 12 flags 0x02 dlen 18  [hci1] 12.152160
      L2CAP: Configure Response (0x05) ident 6 len 10
        Source CID: 64
        Flags: 0x0000
        Result: Success (0x0000)
        Option: Maximum Transmission Unit (0x01) [mandatory]
          MTU: 48
< HCI Command: Host Number of Completed Packets (0x03|0x0035) plen 5  [hci1] 12.158806
        Num handles: 1
        Handle: 12
        Count: 1
< ACL Data TX: Handle 12 flags 0x00 dlen 14  [hci1] 12.161822
      L2CAP: Configure Response (0x05) ident 4 len 6
        Source CID: 64
        Flags: 0x0000
        Result: Success (0x0000)
< HCI Command: Host Number of Completed Packets (0x03|0x0035) plen 5  [hci1] 12.163197
        Num handles: 1
        Handle: 12
        Count: 1

Change-Id: I7919fe180cf1d761023bb1478d8e1fb2d21a94d9
Signed-off-by: Arkadiusz Lichwa <arkadiusz.lichwa@tieto.com>