Bluetooth: L2CAP: Move bt_l2cap_br_fixed_chan_register() to l2cap_br.c

Moves, specific for BR/EDR transport, register fixed channel handler
to l2cap_br.c file and make it local. Then using it register BR/EDR
signal channel (CID 1) context during channel layer initialization
in 'bt_l2cap_br_init'.

Change-Id: I5e23fc5d43ae3d7838ecb44ad4fa631eccc8cb4c
Signed-off-by: Arkadiusz Lichwa <arkadiusz.lichwa@tieto.com>