MAINTAINERS: Change list for ioc_serial to linux-serial.

IOC3 is also being used on SGI MIPS systems but this particular driver is
only being used on IA64 systems so linux-mips made no sense as a list.  Pat
also thinks linux-serial@vger.kernel.org is the better list.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>