ethtool: Clarify use of size field for ETHTOOL_GRXFHINDIR

In order to find out the device's RX flow hash table size, ethtool
initially uses ETHTOOL_GRXFHINDIR with a buffer size of zero.  This
must be supported, but it is not necessary to support any other user
buffer size less than the device table size.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>