mlxsw: thermal: Provide optimization for QSFP modules number detection

Use new field "num_of_modules" of MGPIR register for "thermal" interface
in order to get the number of modules supported by system directly from
the system configuration, instead of getting it from port to module
mapping info.

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