iwlwifi: iwl-drv: Fix fall-through warnings for Clang

In preparation to enable -Wimplicit-fallthrough for Clang, fix a
warning by replacing a /* fall through */ comment with the new
pseudo-keyword macro fallthrough; instead of letting the code fall
through to the next case.

Notice that Clang doesn't recognize /* fall through */ comments as
implicit fall-through markings.

Link: https://github.com/KSPP/linux/issues/115Signed-off-by: NGustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: NKalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/edd98d194bfc98b4be93a9bdc303630b719c0e66.1605896060.git.gustavoars@kernel.org

drivers/net/wireless/intel/iwlwifi/iwl-drv.c

@@ -1524,7 +1524,7 @@ static void iwl_req_fw_callback(const struct firmware *ucode_raw, void *context)
 		break;
 	default:
 		WARN(1, "Invalid fw type %d\n", fw->type);
-		/* fall through */
+		fallthrough;
 	case IWL_FW_MVM:
 		op = &iwlwifi_opmode_table[MVM_OP_MODE];
 		break;