sh_eth: add support for changing MTU

The hardware supports the MTU to be changed and the driver it self is
somewhat prepared to support this. This patch hooks up the callbacks to
be able to change the MTU from user-space.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Acked-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: David S. Miller <davem@davemloft.net>