net: smsc95xx: Fix MTU range

The commit f77f0aee ("net: use core MTU range checking in USB NIC
drivers") introduce a common MTU handling for usbnet. But it's missing
the necessary changes for smsc95xx. So set the MTU range accordingly.

This patch has been tested on a Raspberry Pi 3.

Fixes: f77f0aee

 ("net: use core MTU range checking in USB NIC drivers")
Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: David S. Miller <davem@davemloft.net>