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@xxxxxxxxxxxxx>