net: dsa: microchip: KSZ9893: do not write to not supported Output Clock Control Register

This issue was detected after adding regmap register access validation.
KSZ9893 compatible chips do not have "Output Clock Control Register
0x0103". So, avoid writing to it.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>