V4L/DVB (13479): cxusb: fix compile warning

Fix this trivial compile warning:

v4l/cxusb.c:1195: warning: unused variable 'n'

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>