generic: kernel 5.4: fix probe error for AR803x PHYs

generic: kernel 5.4: fix probe error for AR803x PHYs

Atheros PHYs using the at803x driver apart from the AR8031/AR8033 fail
to probe with kernel 5.4, due to ret in at803x_probe being
uninitialized.

[    1.403461] Atheros 8035 ethernet: probe of 4ef600c00.ethernet:01
	       failed with error -1066114012

Initialize ret in order to successfully prove the PHYs on kernel 5.4.

Kernel 5.10 is not affected, as the ret is always assigned prior to
returning.

Tested on OCEDO Koala.

Reported-by: Russell Senior <russell@personaltelco.net>
Signed-off-by: David Bauer <mail@david-bauer.net>