net: bridge: Add checks for enabling the STP.

It is not possible to have the MRP and STP running at the same time on the
bridge, therefore add check when enabling the STP to check if MRP is already
enabled. In that case return error.

Reviewed-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: Horatiu Vultur <horatiu.vultur@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>