tty: mxser: Fix build warning introduced by dfc7b837 (Re: linux-next: build warning after merge of the tty.current tree)

Fix build warning at mxser.c introduced by dfc7b837 (tty: mxser: fix
usage of opmode_ioaddr)
Signed-off-by: Matwey V. Kornilov <matwey@sai.msu.ru>
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>