mlxsw: spectrum: Reduce number of supported 802.1D bridges

Resources allocated for these bridges at init time cannot be later used
for other purposes. While current number is supported by the device,
it's mostly theoretical with regards to any real use case, which leads
to poor utilization of device's resources. Solve that by reducing the
number.

The long term plan is to make this value (along with others) user
configurable via devlink and write it to NVRAM, so that it can be used
during the next init. Until then we must hardcode such values.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>