serial: Remove unused port type

PORT_MFD is not in use since commit

	1bd187de ("x86, intel-mid: remove Intel MID specific serial support")

Remove leftover.

Fixes: 1bd187de

 ("x86, intel-mid: remove Intel MID specific serial support")
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>