can: softing: softing_startstop(): fix set but not used variable warning

In the function softing_startstop() the variable error_reporting is
assigned but not used. The code that uses this variable is commented
out. Its stated that the functionality is not finally verified.

To fix the warning:

| drivers/net/can/softing/softing_fw.c:424:9: error: variable 'error_reporting' set but not used [-Werror,-Wunused-but-set-variable]

remove the comment, activate the code, but add a "0 &&" to the if
expression and rely on the optimizer rather than the preprocessor to
remove the code.

Link: https://lore.kernel.org/all/20220109103126.1872833-1-mkl@pengutronix.de
Fixes: 03fd3cf5 ("can: add driver for Softing card")
Cc: Kurt Van Dijck <dev.kurt@vandijck-laurijssen.be>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

drivers/net/can/softing/softing_fw.c

@@ -565,18 +565,19 @@ int softing_startstop(struct net_device *dev, int up)
 		if (ret < 0)
 			goto failed;
 	}
-	/* enable_error_frame */
-	/*
+
+	/* enable_error_frame
+	 *
 	 * Error reporting is switched off at the moment since
 	 * the receiving of them is not yet 100% verified
 	 * This should be enabled sooner or later
-	 *
-	if (error_reporting) {
+	 */
+	if (0 && error_reporting) {
 		ret = softing_fct_cmd(card, 51, "enable_error_frame");
 		if (ret < 0)
 			goto failed;
 	}
-	*/
+
 	/* initialize interface */
 	iowrite16(1, &card->dpram[DPRAM_FCT_PARAM + 2]);
 	iowrite16(1, &card->dpram[DPRAM_FCT_PARAM + 4]);