net: ethernet: aquantia: Fixes for aq_ndev_change_mtu

1)Removed unnecessary comparsion "old_mtu == new_mtu".
This check is not needed. Function aq_ndev_change_mtu wont be called
if mtu has not changed.

2)Removed extra assignment ndev->mtu = new_mtu;
This assignment already done inside __dev_set_mtu().

3)Use core MTU checking for min_mtu.

Signed-off-by: Pavel Belous <pavel.belous@aquantia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>