[SERIAL] Fix moxa tty driver name

The moxa driver was named "ttya", which is wrong:
1) Documentation/devices.txt says that the name should be "ttyMX".
2) First 10 ports (ttya0...ttya9) clash with the legacy pty driver.

This patch changes the driver name to "ttyMX".

http://bugme.osdl.org/show_bug.cgi?id=5012

Signed-off-by: Sergey Vlasov <vsu@altlinux.ru>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>