cxgb4: Update comment for min_mtu

We have lost a comment for minimum mtu value set for netdevice with
'commit d894be57

 ("ethernet: use net core MTU range checking in
more drivers"). Updating it accordingly.

Signed-off-by: Arjun Vynipadath <arjun@chelsio.com>
Signed-off-by: Ganesh Goudar <ganeshgr@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>