ravb: add support for changing MTU

Allow for changing the MTU within the limit of the maximum size of a
descriptor (2048 bytes). Add the callback to change MTU from user-space
and take the configurable MTU into account when configuring the
hardware.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: David S. Miller <davem@davemloft.net>