net: dsa: mv88e6xxx: Fix mv88e6352_serdes_get_stats error path

net: dsa: mv88e6xxx: Fix mv88e6352_serdes_get_stats error path

[ Upstream commit fc82a08ae795ee6b73fb6b50785f7be248bec7b5 ]

mv88e6xxx_get_stats, which collects stats from various sources,
expects all callees to return the number of stats read. If an error
occurs, 0 should be returned.

Prevent future mishaps of this kind by updating the return type to
reflect this contract.

Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Signed-off-by: Tobias Waldekranz <tobias@waldekranz.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>