ieee1394: eth1394: allow MTU bigger than 1500

RFC 2734 says: "IP-capable nodes may operate with an MTU size larger
than the default [1500 octets], but the means by which a larger MTU is
configured are beyond the scope of this document."

Allow users to set an MTU bigger than 1500.

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>