staging: mt7621-eth: fix compile warning.

This code generates a waring as PHY_GBIT_FEATURES is "long" but
->supported in "int".

It looks likely that "PHY_1000BT_FEATURES" is the correct define to
use - it is intended to be used with the ->features field.

Signed-off-by: NeilBrown <neil@brown.name>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>