iwlwifi: mvm: reset the fw_dump_desc pointer after ASSERT

When we get an ASSERT, the fw_dump_desc pointer points to
iwl_mvm_dump_desc_assert which can't be freed since it is
a global. We still need to NULL'ify the pointer when we
call iwl_mvm_free_fw_dump_desc otherwise we will hit

int iwl_mvm_fw_dbg_collect_desc(struct iwl_mvm *mvm,
                                const struct iwl_mvm_dump_desc *desc,
                                const struct iwl_fw_dbg_trigger_tlv *trigger)
{
<snip>
        if (WARN_ON(mvm->fw_dump_desc))
                iwl_mvm_free_fw_dump_desc(mvm);

Fixes: b6eaa45a ("iwlwifi: mvm: add the cause of the firmware dump in the dump")
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>