Bluetooth: HFP HF: SLC connection-Send/Parse BRSF

Service Level Connection(SLC) Initialization part. Sending AT+BRSF
and parsing its response.

< ACL Data TX: Handle 256 flags 0x00 dlen 20
      Channel: 75 len 16 [PSM 3 mode 0] {chan 0}
      RFCOMM: Unnumbered Info with Header Check (UIH) (0xef)
         Address: 0x09 cr 0 dlci 0x02
         Control: 0xef poll/final 0
         Length: 12
         FCS: 0x40
        41 54 2b 42 52 53 46 3d 31 34 38 0d 40           AT+BRSF=20.

.
> ACL Data RX: Handle 256 flags 0x02 dlen 23
      Channel: 64 len 19 [PSM 3 mode 0] {chan 0}
      RFCOMM: Unnumbered Info with Header Check (UIH) (0xef)
         Address: 0x0b cr 1 dlci 0x02
         Control: 0xff poll/final 1
         Length: 14
         FCS: 0x86
         Credits: 4
        0d 0a 2b 42 52 53 46 3a 20 38 37 31 0d 0a 86     ..+BRSF: 871.
> ACL Data RX: Handle 256 flags 0x02 dlen 14
      Channel: 64 len 10 [PSM 3 mode 0] {chan 0}
      RFCOMM: Unnumbered Info with Header Check (UIH) (0xef)
         Address: 0x0b cr 1 dlci 0x02
         Control: 0xef poll/final 0
         Length: 6
         FCS: 0x9a
        0d 0a 4f 4b 0d 0a 9a                             ..OK...

Change-Id: I51581928d479ea4229b32a07cbea86c1f6fe09c8
Signed-off-by: Sathish Narasimman <sathish.narasimman@intel.com>