net: dsa: pass bridge down to drivers

Some DSA drivers may or may not support multiple software bridges on top
of an hardware switch.

It is more convenient for them to access the bridge's net_device for
finer configuration.

Removing the need to craft and access a bitmask also simplifies the
code.

This patch changes the signature of bridge related functions, update DSA
drivers, and removes dsa_slave_br_port_mask.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Tested-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>