mlxsw: spectrum: Add sanity checks into module info get

Driver assumes certain values in the PMLP register. Add checks that
verify that PMLP register provides fitting values.

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