mlxsw: spectrum_cnt: Query bank size from FW resources

The bank size is different between Spectrum versions. Also it is
a resource that can be queried. So instead of hard coding the value in
code, query it from the firmware.

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