ALSA: timer: Adjust a condition check in snd_timer_resolution()

The script "checkpatch.pl" pointed information out like the following.

ERROR: do not use assignment in if condition

Thus fix the affected source code place.
Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/core/timer.c

@@ -393,7 +393,8 @@ unsigned long snd_timer_resolution(struct snd_timer_instance *timeri)
 
 	if (timeri == NULL)
 		return 0;
-	if ((timer = timeri->timer) != NULL) {
+	timer = timeri->timer;
+	if (timer) {
 		if (timer->hw.c_resolution)
 			return timer->hw.c_resolution(timer);
 		return timer->hw.resolution;