net: dsa: sja1105: Fix assigned yet unused return code rc

The return code variable rc is being set to return error values in two
places in sja1105_mdiobus_base_tx_register and yet it is not being
returned, the function always returns 0 instead. Fix this by replacing
the return 0 with the return code rc.

Addresses-Coverity: ("Unused value")
Fixes: 5a8f0974 ("net: dsa: sja1105: register the MDIO buses for 100base-T1 and 100base-TX")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>