mlxsw: core: Remove unnecessary asserts

Remove unnecessary asserts for module index validation. Leave only one
that is actually necessary in mlxsw_env_pmpe_listener_func() where the
module index is directly read from the firmware event.

Signed-off-by: Vadim Pasternak <vadimp@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>