net: dsa: microchip: improving error handling for 8-bit register RMW operations

This patch refines the error handling mechanism for 8-bit register
read-modify-write operations. In case of a failure, it now logs an error
message detailing the problematic offset. This enhancement aids in
debugging by providing more precise information when these operations
encounter issues.

Furthermore, the ksz_prmw8() function has been updated to return error
values rather than void, enabling calling functions to appropriately
respond to errors.

Additionally, in case of an error that affects both the current and
future accesses, the PHY driver will log the errors consistently, akin
to the existing behavior in all ksz_read*/ksz_write* helpers.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>