Add a new registration function to register the PCMCIA 16-bit subsystem
(ds a.k.a. pcmcia) with the PCMICA core (cs a.k.a. pcmcia_core).
As send_event is only called with skt->sem held, we can use that to safeguard
skt->callback(), too. Note that the class_device_register() call by pccardd()
is done _before_ skt->sem() is held, and the pcmcia_socket_register() doesn't
hold skt->sem() as well, so there is no chance for a deadlock.