net: phy: avoid suspending twice a PHY

As part of a call to ndo_close() a netdevice driver may call
phy_disconnect() -> phy_detach() -> phy_suspend(), such that the PHY is
suspsended at this point and a netdevice driver may clock gate the
backing peripheral providing MDIO bus accessses as well.

Update mdio_bus_phy_may_suspend() to return whether a PHY is allowed to
be suspended and conversely resumed if and only if it was not previously
suspended before while it is currently in detached (netdev pointer is
NULL) state.

This fixes bus errors seen during S2/S3 suspend/resume cycles for
netdevice drivers such as GENET which clock gates the entire Ethernet
MAC, including the MDIO bus block.

Acked-by: Fugang Duan <B38611@freescale.com>
Tested-by: Fugang Duan <B38611@freescale.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>