serial: fix enable_irq_wake/disable_irq_wake imbalance in serial_core.c

enable_irq_wake() and disable_irq_wake() need to be balanced.  However,
serial_core.c calls these for different conditions during the suspend and
resume functions...

This is causing a regular WARN_ON() as found at
http://www.kerneloops.org/search.php?search=set_irq_wake

This patch makes the conditions for triggering the _wake enable/disable
sequence identical.

Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>