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>