alx: add feature flag for rx checksumming

The code to handle rx checksumming was in the driver since its introduction
but for reasons unknown the feature flag was left out. Now it is possible
to enable this feature with ethtool.

Tested on my AR8161 ethernet card, there are no regressions observed in
netperf if this feature is enabled.

Signed-off-by: Tobias Regnery <tobias.regnery@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>