char: nwbutton: avoid unused variable warning

When CONFIG_NWBUTTON_REBOOT is disabled, we get a warning about
an unused variable:

drivers/char/nwbutton.c:37:12: warning: 'reboot_count' defined but not used [-Wunused-variable]
 static int reboot_count = NUM_PRESSES_REBOOT; /* Number of presses to reboot */

Using if(IS_ENABLED()) instead of #ifdef around the user makes the
code nicer to read and avoids the warning.
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/char/nwbutton.c

@@ -129,10 +129,9 @@ static void button_consume_callbacks (int bpcount)
 
 static void button_sequence_finished (unsigned long parameters)
 {
-#ifdef CONFIG_NWBUTTON_REBOOT		/* Reboot using button is enabled */
-	if (button_press_count == reboot_count)
+	if (IS_ENABLED(CONFIG_NWBUTTON_REBOOT) &&
+	    button_press_count == reboot_count)
 		kill_cad_pid(SIGINT, 1);	/* Ask init to reboot us */
-#endif /* CONFIG_NWBUTTON_REBOOT */
 	button_consume_callbacks (button_press_count);
 	bcount = sprintf (button_output_buffer, "%d\n", button_press_count);
 	button_press_count = 0;		/* Reset the button press counter */