net/cadence/macb: fix invalid 0 return if no phy is discovered on mii init

Replace misleading -1 (-EPERM) by a more appropriate return code (-ENXIO)
in macb_mii_probe function.
Save macb_mii_probe return before branching to err_out_unregister to avoid
erronous 0 return.

Signed-off-by: Boris BREZILLON <b.brezillon@overkiz.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>