net: dsa: microchip: Fix KSZ8863 reset problem

net: dsa: microchip: Fix KSZ8863 reset problem

ksz8873_valid_regs[] was added for register access for KSZ8863/KSZ8873
switches, but the reset register is not in the list so
ksz8_reset_switch() does not take any effect.

Replace regmap_update_bits() using ksz_regmap_8 with ksz_rmw8() so that
an error message will be given if the register is not defined.

A side effect of not resetting the switch is the static MAC table is not
cleared.  Further additions to the table will show write error as there
are only 8 entries in the table.

Fixes: d0dec3333040 ("net: dsa: microchip: Add register access control for KSZ8873 chip")
Signed-off-by: Tristram Ha <tristram.ha@microchip.com>
Reviewed-by: Oleksij Rempel <o.rempel@pengutronix.de>
Link: https://patch.msgid.link/20250807005453.8306-1-Tristram.Ha@microchip.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>