net: phy: fixed_phy: Fix fixed_phy not checking GPIO

Fix fixed_phy not checking GPIO if no link_update callback
is registered.

In the original version all users registered a link_update
callback so the issue was masked.

Fixes: a5597008 ("phy: fixed_phy: Add gpio to determine link up/down.")
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Moritz Fischer <mdf@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>