net: dsa: microchip: Remove redundant optimization in ksz8_w_phy_bmcr

Remove the manual checks for register value changes in the
ksz8_w_phy_bmcr function. Instead, rely on regmap_update_bits() for
optimizing register updates.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Acked-by: Arun Ramadoss <arun.ramadoss@microchip.com>
Link: https://lore.kernel.org/r/20240124123314.734815-3-o.rempel@pengutronix.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>