net: Define eth_stp_addr in linux/etherdevice.h

The lan9303 driver defines eth_stp_addr as a synonym to
eth_reserved_addr_base to get the STP ethernet address 01:80:c2:00:00:00.

eth_reserved_addr_base is also used to define the start of Bridge Reserved
ethernet address range, which happen to be the STP address.

br_dev_setup refer to eth_reserved_addr_base as a definition of STP
address.

Clean up by:
 - Move the eth_stp_addr definition to linux/etherdevice.h
 - Use eth_stp_addr instead of eth_reserved_addr_base in br_dev_setup.

Signed-off-by: Egil Hjelmeland <privat@egil-hjelmeland.no>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>