stmmac: fix pm functions avoiding sleep on spinlock

This patch fixes the pm functions to avoid the system
sleeps while a spinlock is taken.

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