stmmac: fix suspend/resume locking

Upon resume from standby, there is a possible interrupt
unsafe locking scenario raised when configure the Kernel
with CONFIG_PROVE_LOCKING. So this patch fixes that in
PM driver stuff by calling lock/unlock_irqsave/restore.

Signed-off-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>