net: dsa: use the ETH_MIN_MTU and ETH_DATA_LEN default values

Now that DSA supports MTU configuration, undo the effects of commit
8b1efc0f ("net: remove MTU limits on a few ether_setup callers") and
let DSA interfaces use the default min_mtu and max_mtu specified by
ether_setup(). This is more important for min_mtu: since DSA is
Ethernet, the minimum MTU is the same as of any other Ethernet
interface, and definitely not zero. For the max_mtu, we have a callback
through which drivers can override that, if they want to.

Signed-off-by: Vladimir Oltean <olteanv@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>