net: phy: smsc: Cope with hot-removal in interrupt handler

If reading the Interrupt Source Flag register fails with -ENODEV, then
the PHY has been hot-removed and the correct response is to bail out
instead of throwing a WARN splat and attempting to suspend the PHY.
The PHY should be stopped in due course anyway as the kernel
asynchronously tears down the device.

Tested-by: Oleksij Rempel <o.rempel@pengutronix.de> # LAN9514/9512/9500
Tested-by: Ferry Toth <fntoth@gmail.com> # LAN9514
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>