CAPI: Dynamically register minor devices

Register capiminors dynamically with the TTY core so that udev can make
them show up as the NCCIs appear or disappear. This removes the need to
check if the capiminor requested in capinc_tty_open actually exists.

And this completely obsoletes capifs which will be scheduled for removal
in a later patch.

Signed-off-by: Jan Kiszka <jan.kiszka@web.de>
Signed-off-by: David S. Miller <davem@davemloft.net>