stmmac: fix stmmac_resume removing not yet used shutdown flag

The commit to convert to use the dev_pm_ops struct
introduces a bug. The shutdown flag is not yet used
because the hibernation on memory is done by using
the freeze callback.
Thanks to Vlad for having reported it.

Reported-by: Vlad Lungu <vlad.lungu@windriver.com>
Signed-off-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>