staging: mt7621-eth: Fix sparse warning in ethtool.c

This fixes the following sparse warning:

drivers/staging/mt7621-eth/ethtool.c:213:6: warning: symbol
'mtk_set_ethtool_ops' was not declared. Should it be static?

Signed-off-by: Chris Coffey <cmc@babblebit.net>
Reviewed-by: NeilBrown <neil@brown.name>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>