ionic: support ethtool rxhash disable

We can disable rxhashing by setting rss_types to 0.  The user
can toggle this with "ethtool -K <ethX> rxhash off|on",
which calls into the .ndo_set_features callback with the
NETIF_F_RXHASH feature bit set or cleared.  This patch adds
a check for that bit and updates the FW if necessary.

Signed-off-by: Shannon Nelson <snelson@pensando.io>
Signed-off-by: David S. Miller <davem@davemloft.net>