[PATCH] ioremap balanced with iounmap for drivers/serial/mux.c

ioremap must be balanced by an iounmap and failing to do so can result
in a memory leak.

Signed-off-by: Amol Lad <amol@verismonetworks.com>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>