net: ksz884x: convert to hw_features

This also fixes possible race when changing receive checksum state
and removes IPV6_CSUM_GEN_HACK as it's always set.

BTW, The claim about fake IPV6 checksum looks dubious. If that were true,
then there's a problem in networking core and should be fixed there and not
in random drivers.

BTW#2, there's no MAINTAINERS entry for this driver. I assume this driver
is supported by Micrel?

Signed-off-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Signed-off-by: David S. Miller <davem@davemloft.net>