serial_txx9: Use UPF_FIXED_PORT

The UPF_FIXED_PORT flags was introduced in 2.6.22 and it can be used
instead of the driver specific verify_port routine.

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>