gianfar: Make reset_gfar static

Fix sparse warning:

drivers/net/ethernet/freescale/gianfar.c:2070:6:
 warning: symbol 'reset_gfar' was not declared. Should it be static?

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>