Bluetooth: Fix sending incorrect MTU in the Exchange MTU response

According to the Core Spec 4.2 Vol 3 Part G
The server shall respond with the Server Rx MTU parameter set to the
maximum MTU that this server can receive.
Once the messages have been exchanged, the ATT_MTU shall be set
to the minimum of the Client Rx MTU and Server Rx MTU values.

bt: att_mtu_req (0x0010fd04): Client MTU 672
bt: att_mtu_req (0x0010fd04): Server MTU 65
bt: att_mtu_req (0x0010fd04): Negotiated MTU 65

bt: att_mtu_req (0x0010fd04): Client MTU 42
bt: att_mtu_req (0x0010fd04): Server MTU 65
bt: att_mtu_req (0x0010fd04): Negotiated MTU 42

Change-Id: I13f2f0fc99e99d8188ed15bf7972a9b892612e11
Signed-off-by: Mariusz Skamra <mariusz.skamra@tieto.com>