Bluetooth: Get rid of __rfcomm_get_sock_by_channel()

rfcomm_get_sock_by_channel() was the only user of this function, so I merged
both into rfcomm_get_sock_by_channel(). The socket lock now should be hold
outside of rfcomm_get_sock_by_channel() once we hold and release it inside the
same function now.

Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>