fjes: Enhance changing MTU related work

This patch enhances the fjes_change_mtu() method
by introducing new flag named FJES_RX_MTU_CHANGING_DONE
in rx_status. At the same time, default MTU value is
changed into 65510 bytes.

Signed-off-by: Taku Izumi <izumi.taku@jp.fujitsu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>