PM: Fix misuse of wakeup flag accessors in serial core

This patch (as1059) fixes a mistake in the way the serial core
initializes a device's wakeup settings.  It should use the accessor
routine instead of relying on a macro producing an lvalue.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>