net: ethernet: aquantia: set net_device mtu when mtu is changed

When the aquantia device mtu is changed the net_device structure is not
updated.  As a result the ip command does not properly reflect the mtu change.

Commit 5513e164 incorrectly assumed that __dev_set_mtu() was making the
assignment ndev->mtu = new_mtu;  This is not true in the case where the driver
has a ndo_change_mtu routine.

Fixes: 5513e164 ("net: ethernet: aquantia: Fixes for aq_ndev_change_mtu")

Cc: Pavel Belous <Pavel.Belous@aquantia.com>
Signed-off-by: David Arcari <darcari@redhat.com>
Tested-by: Pavel Belous <pavel.belous@aquantia.com>
Reviewed-by: Jarod Wilson <jarod@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>