ethernet/atheros: use core min/max MTU checking

atl2: min_mtu 40, max_mtu 1504

- Remove a few redundant defines that already have equivalents in
  if_ether.h.

atl1: min_mtu 42, max_mtu 10218

atl1e: min_mtu 42, max_mtu 8170

atl1c: min_mtu 42, max_mtu 6122/1500

- GbE hardware gets a max_mtu of 6122, slower hardware gets 1500.

alx: min_mtu 34, max_mtu 9256

- Not so sure that minimum MTU number is really what was intended, but
  that's what the math actually makes it out to be, due to max_frame
  manipulations and comparison in alx_change_mtu, rather than just
  comparing new_mtu. (I think 68 was the intended min_mtu value).

CC: netdev@vger.kernel.org
CC: Jay Cliburn <jcliburn@gmail.com>
CC: Chris Snook <chris.snook@gmail.com>
Signed-off-by: Jarod Wilson <jarod@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>