tty: mxser: local variables should not be exposed globally

The variable 'mxser_port_ops' is only referenced in this file and
should be marked static to prevent it from being exposed globally.

Quites the sparse warning:

warning: symbol 'mxser_port_ops' was not declared. Should it be static?

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Jiri Slaby <jirislaby@gmail.com>
Acked-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>