media: cx88: add error handling for snd_ctl_add

When snd_ctl_add fails, the lack of error-handling code may
cause unexpected results.

This patch adds error-handling code after calling snd_ctl_add.

Signed-off-by: Zhouyang Jia <jiazhouyang09@gmail.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>