[media] ivtv/cx18: fix compiler warnings

media_build/v4l/cx18-alsa-main.c: In function 'cx18_alsa_exit':
media_build/v4l/cx18-alsa-main.c:282:6: warning: variable 'ret' set but not used [-Wunused-but-set-variable]
media_build/v4l/cx18-mailbox.c: In function 'cx18_api_call':
media_build/v4l/cx18-mailbox.c:598:6: warning: variable 'state' set but not used [-Wunused-but-set-variable]
media_build/v4l/cx18-alsa-pcm.c: In function 'snd_cx18_pcm_capture_open':
media_build/v4l/cx18-alsa-pcm.c:156:6: warning: variable 'ret' set but not used [-Wunused-but-set-variable]
media_build/v4l/cx18-alsa-pcm.c: In function 'snd_cx18_pcm_capture_close':
media_build/v4l/cx18-alsa-pcm.c:202:6: warning: variable 'ret' set but not used [-Wunused-but-set-variable]
media_build/v4l/cx18-alsa-pcm.c: In function 'snd_cx18_pcm_hw_params':
media_build/v4l/cx18-alsa-pcm.c:255:6: warning: variable 'ret' set but not used [-Wunused-but-set-variable]
media_build/v4l/cx18-streams.c: In function 'cx18_stop_v4l2_encode_stream':
media_build/v4l/cx18-streams.c:983:16: warning: variable 'then' set but not used [-Wunused-but-set-variable]
media_build/v4l/ivtv-ioctl.c: In function 'ivtv_set_speed':
media_build/v4l/ivtv-ioctl.c:138:22: warning: variable 's' set but not used [-Wunused-but-set-variable]
media_build/v4l/ivtvfb.c: In function 'ivtvfb_init':
media_build/v4l/ivtvfb.c:1286:6: warning: variable 'err' set but not used [-Wunused-but-set-variable]
media_build/v4l/ivtvfb.c: In function 'ivtvfb_cleanup':
media_build/v4l/ivtvfb.c:1306:6: warning: variable 'err' set but not used [-Wunused-but-set-variable]

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>