aquantia: Setup max_mtu in ndev to enable jumbo frames

Although hardware is capable for almost 16K MTU, without max_mtu field
correctly set it only allows standard MTU to be used.
This patch enables max MTU, calculating it from hardware maximum frame size
of 16352 octets (including FCS).

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

Signed-off-by: Pavel Belous <Pavel.Belous@aquantia.com>
Signed-off-by: Igor Russkikh <igor.russkikh@aquantia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>