phy: add proper phy struct device refcounting

Take a refcount on the phy struct device when the phy device is attached
to a network device, and drop it after it's detached.  This ensures that
a refcount is held on the phy device while the device is being used by
a network device, thereby preventing the phy_device from being
unexpectedly kfree()'d by phy_device_release().

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>