net: dsa: microchip: get P_STP_CTRL in ksz_port_stp_state by ksz_dev_ops

At present, P_STP_CTRL register value is passed as parameter to
ksz_port_stp_state from the individual dsa_switch_ops hooks. This patch
update the function to retrieve the register value through the
ksz_chip_data member.
And add the static to ksz_update_port_member since it is not called
outside the ksz_common.

Signed-off-by: Arun Ramadoss <arun.ramadoss@microchip.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>