MN10300: Fix RTC routines

A change to the RTC routines in the MN10300 arch used set_rtc_mms() when it
meant set_rtc_mmss().  This results in an error due to a reference of an
undefined symbol.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>