net: dsa: microchip: add ksz_rmw8() function

[ Upstream commit 6f1b986a ]

Add ksz_rmw8(), it will be used in the next patch.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Acked-by: Arun Ramadoss <arun.ramadoss@microchip.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Stable-dep-of: bb986904 ("net: dsa: microchip: Fix LAN937X set_ageing_time function")
Signed-off-by: Sasha Levin <sashal@kernel.org>