net: dsa: microchip: add P_STP_CTRL to ksz_chip_reg

This patch moves the stp_ctrl_reg from the ksz_chip_data to ksz_chip_reg
structure.

Signed-off-by: Arun Ramadoss <arun.ramadoss@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>