serial: fix serial_txx9 console initialization

Since commit 97d97224 ("[SERIAL] Fix
console initialisation ordering"), serial_core calls ->pm() on
initialization even if the port was used for console.

This behaviour breaks serial_txx9 console since The serial_txx9 driver
initialize its port entirely on its ->pm() method if new state was 0.
This patch adds checking for oldstate value to fix this probelm.

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>