MN10300: Fix MN10300's serial port driver to get at its tty_struct

Fix MN10300's serial port driver to get at its tty_struct as this moved
from struct uart_info into struct tty_port in patch:

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