stmmac: do not sleep in atomic context for mdio_reset

stmmac_mdio_reset() has been updated to use msleep rather udelay
(as some PHY requires a one second delay there).
It called from stmmac_resume() within the spin_lock_irqsave block
atomic context triggering 'scheduling while atomic'.

The stmmac_priv lock usage is not fully documented, but it seems
to protect the access to the MAC registers / DMA structures rather
than the MDIO bus or the PHY (which have separate locking),
so we can push the spin_lock after the stmmac_mdio_reset call.

Signed-off-by: Vincent Palatin <vpalatin@chromium.org>
Signed-off-by: David S. Miller <davem@davemloft.net>