mtk-star-emac: mark PM functions as __maybe_unused

Without CONFIG_PM, the compiler warns about two unused functions:

drivers/net/ethernet/mediatek/mtk_star_emac.c:1472:12: error: unused function 'mtk_star_suspend' [-Werror,-Wunused-function]
drivers/net/ethernet/mediatek/mtk_star_emac.c:1488:12: error: unused function 'mtk_star_resume' [-Werror,-Wunused-function]

Mark these as __maybe_unused.

Fixes: 8c7bd5a4 ("net: ethernet: mtk-star-emac: new driver")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Signed-off-by: David S. Miller <davem@davemloft.net>