phy: ti: gmii-sel: Always write the RGMII ID setting

Some SoCs are just validated with the TX delay enabled. With commit
ca13b249 ("net: ethernet: ti: am65-cpsw: fixup PHY mode for fixed
RGMII TX delay"), the network driver will patch the delay setting on the
fly assuming that the TX delay setting is fixed. In reality, the TX
delay is configurable and just skipped in the documentation. There are
bootloaders, which will disable the TX delay and this will lead to a
transmit path which doesn't add any delays at all.
Fix that by always writing the RGMII_ID setting and report an error for
unsupported RGMII delay modes.

This is safe to do and shouldn't break any boards in mainline because
the fixed delay is only introduced for gmii-sel compatibles which are
used together with the am65-cpsw-nuss driver and also contains the
commit above.

Fixes: ca13b249 ("net: ethernet: ti: am65-cpsw: fixup PHY mode for fixed RGMII TX delay")
Signed-off-by: Michael Walle <mwalle@kernel.org>
Reviewed-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Link: https://lore.kernel.org/r/20250819065622.1019537-1-mwalle@kernel.org


Signed-off-by: Vinod Koul <vkoul@kernel.org>