mlxsw: spectrum: Use PMTM register to get max module width

Currently the max module width is hard-coded according to ASIC type.
That is not entirely correct, as the max module width might differ
per-board. Use PMTM register to query FW for maximal width of a module.

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>