Bluetooth: Move parts of fixed channel initialization to l2cap_add_scid

The l2cap_add_scid function is used for registering a fixed L2CAP
channel. Instead of having separate initialization of the channel type
and outgoing MTU in l2cap_sock.c it's more intuitive to do these things
in the l2cap_add_scid function itself (and thereby make the
functionality available to other users besides l2cap_sock.c).

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>