iwlwifi: mvm: fix memory leaks in error paths upon fw error dump

When iwl_mvm_fw_error_dump fails, it needs to clear the
state in mvm, which includes:
	* clear IWL_MVM_STATUS_DUMPING_FW_LOG
	* set mvm->fw_dump_trig to NULL
	* free the description

While at it, remove a NULL check in
iwl_mvm_free_fw_dump_desc since kfree is NULL safe.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>