MN10300: Fix up the IRQ names for the on-chip serial ports

Fix up the IRQ names for the MN10300 on-chip serial ports in the driver as
request_interrupt() no longer allows names containing slashes, giving a warning
like the following if one is encountered:

	------------[ cut here ]------------
	WARNING: at fs/proc/generic.c:323 __xlate_proc_name+0x62/0x7c()
	name 'ttySM0/Rx'

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