net: dsa: microchip: ksz8_r_sta_mac_table(): Avoid using error code for empty entries

[ Upstream commit 559901b4 ]

Prepare for the next patch by ensuring that ksz8_r_sta_mac_table() does
not use error codes for empty entries. This change will enable better
handling of read/write errors in the upcoming patch.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Stable-dep-of: 4bdf79d6 ("net: dsa: microchip: correct KSZ8795 static MAC table access")
Signed-off-by: Sasha Levin <sashal@kernel.org>