net: stmmac: fix missing unlock on error in stmmac_suspend()

commit 30f347ae upstream.

Add the missing unlock before return from stmmac_suspend()
in the error handling case.

Fixes: 5ec55823 ("net: stmmac: add clocks management for gmac driver")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Yongxin Liu <yongxin.liu@windriver.com>