atl1c: adjust max mtu according to Mikrotik 10/25G NIC ability

The new Mikrotik 10/25G NIC supports jumbo frames. Jumbo frames are
supported for TSO as well.

This enables the support for mtu up to 9500 bytes.

Signed-off-by: Gatis Peisenieks <gatis@mikrotik.com>
Signed-off-by: David S. Miller <davem@davemloft.net>