mvpp2: refactor MTU change code

The MTU change code can call napi_disable() with the device already down,
leading to a deadlock. Also, lot of code is duplicated unnecessarily.

Rework mvpp2_change_mtu() to avoid the deadlock and remove duplicated code.

Fixes: 3f518509 ("ethernet: Add new driver for Marvell Armada 375 network unit")
Signed-off-by: Matteo Croce <mcroce@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>