tty: relock generic_serial

Switch generic_serial to do port count locking via the tty_port structure
ready for moving to a common port wait routine. Keep the old driver lock for
internal calling so we don't risk messing up the drivers below until we
are ready.

Still needs kref conversions

Signed-off-by: Alan Cox <alan@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>