pasemi_mac: add support for setting MTU

Currently keeping it at 1500 bytes or below since jumbo frames need
special checksum offload on TX.

Signed-off-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Jeff Garzik <jeff@garzik.org>
Signed-off-by: David S. Miller <davem@davemloft.net>