ALSA: usb-audio: Remove set but not used variable 'first_ch_bits'

Fixes gcc '-Wunused-but-set-variable' warning:

sound/usb/mixer.c: In function 'parse_audio_feature_unit':
sound/usb/mixer.c:1838:28: warning:
 variable 'first_ch_bits' set but not used [-Wunused-but-set-variable]

It never used since 2.6
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/usb/mixer.c

@@ -1835,7 +1835,7 @@ static int parse_audio_feature_unit(struct mixer_build *state, int unitid,
 {
 	int channels, i, j;
 	struct usb_audio_term iterm;
-	unsigned int master_bits, first_ch_bits;
+	unsigned int master_bits;
 	int err, csize;
 	struct uac_feature_unit_descriptor *hdr = _ftr;
 	__u8 *bmaControls;
@@ -1926,10 +1926,6 @@ static int parse_audio_feature_unit(struct mixer_build *state, int unitid,
 		break;
 
 	}
-	if (channels > 0)
-		first_ch_bits = snd_usb_combine_bytes(bmaControls + csize, csize);
-	else
-		first_ch_bits = 0;
 
 	if (state->mixer->protocol == UAC_VERSION_1) {
 		/* check all control types */