Bluetooth: L2CAP: Refactor l2cap_chan_get() in l2cap.c

Simplify l2cap_chan_get() in l2cap.c file by removing
CONFIG_BLUETOOTH_BREDR preproprocessor check. Now function serves
only getting channel on LE transport.

Change-Id: I20b72f352ad9d5998cc812b28cb3d6951f91f212
Signed-off-by: Arkadiusz Lichwa <arkadiusz.lichwa@tieto.com>