phy: fixed-phy: properly validate phy in fixed_phy_update_state()

Validate that the phy_device passed into fixed_phy_update_state() is a
fixed-phy device before walking the list of phys for a fixed phy at the
same address.

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