mxser: spin_lock() => spin_lock_irq()

This should be spin_lock_irq() to match the spin_unlock_irq().  Originally
it was a lock_kernel() but we switched everything to spin_lock_irq() last
November.

[akpm@linux-foundation.org: fix the MOXA_ASPP_MON case too (per Jiri)]
Signed-off-by: Dan Carpenter <error27@gmail.com>
Cc: Jiri Slaby <jirislaby@gmail.com>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: Greg KH <greg@kroah.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>