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

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>