Staging: generic_serial: fix double locking bug

spin_lock_irqsave() is not nestable.  The second time that it gets
called it overwrites the "flags" variable and so IRQs can't be restored
properly.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>