MAINTAINERS: move atlx discussions to netdev

The atlx drivers are sufficiently mature that we no longer need a separate
mailing list for them.  Move the discussion to netdev, so we can decommission
atl1-devel, which is now mostly spam.

Signed-off-by: Chris Snook <chris.snook@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>