gianfar: Reject out-of-range RX NFC locations

Currently the driver only uses location values to maintain an ordered
list of filters.  Make it reject location values >= MAX_FILER_IDX
passed to the ETHTOOL_SRXCLSRLINS command, consistent with the range
it reports for the ETHTOOL_GRXCLSRLALL command.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Acked-by: Sebastian Pöhn <sebastian.poehn@googlemail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>