net: phy: Check phydev->drv

There are number of function calls, originating from user-space,
typically through the Ethernet driver that can make us crash by
dereferencing phydev->drv which will be NULL once we unbind the driver
from the PHY.

There are still functional issues that prevent an unbind then rebind to
work, but these will be addressed separately.

Suggested-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>