net: dsa: ksz: added the generic port_stp_state_set function

The ksz8795 and ksz9477 uses the same algorithm for the
port_stp_state_set function except the register address is different. So
moved the algorithm to the ksz_common.c and used the dev_ops for
register read and write. This function can also used for the lan937x
part. Hence making it generic for all the parts.

Signed-off-by: Arun Ramadoss <arun.ramadoss@microchip.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/r/20220424112831.11504-1-arun.ramadoss@microchip.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>