mvgbe: remove warning for unused methods

Some baords may use the GBE interface but they have no
phy connected to the interface and a direct MAC/MAC interface.
For them we need to get rid of compiler warnings.

Signed-off-by: Stefan Bigler <stefan.bigler@keymile.com>
Signed-off-by: Valentin Longchamp <valentin.longchamp@keymile.com>
Acked-by: Prafulla Wadaskar <prafulla@marvell.com>