net: dsa: loop: Do not unregister invalid fixed PHY

During error injection it was possible to crash in dsa_loop_exit() because of
an attempt to unregister an invalid PHY. We actually want to the driver probing
in dsa_loop_init() even though fixed_phy_register() may return an error to
exercise how DSA deals with such cases, but we should not be crashing during
driver removal.

Fixes: 98cd1552 ("net: dsa: Mock-up driver")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>