net: bcmgenet: return correct value 'ret' from bcmgenet_power_down

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/net/ethernet/broadcom/genet/bcmgenet.c: In function 'bcmgenet_power_down':
drivers/net/ethernet/broadcom/genet/bcmgenet.c:1136:6: warning:
 variable 'ret' set but not used [-Wunused-but-set-variable]

bcmgenet_power_down should return 'ret' instead of 0.

Fixes: ca8cf341 ("net: bcmgenet: propagate errors from bcmgenet_power_down")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>