ethtool: Declare netdev_rss_key as __read_mostly.

netdev_rss_key is written to once and thereafter is read by
drivers when they are initialising. The fact that it is mostly
read and not written to makes it a candidate for a __read_mostly
declaration.

Signed-off-by: Kim Jones <kim-marie.jones@intel.com>
Signed-off-by: Alan Carey <alan.carey@intel.com>
Acked-by: Rami Rosen <rami.rosen@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>