V4L/DVB (7794): cx88: Fix a warning

drivers/media/video/cx88/cx88-i2c.c: In function 'attach_inform':
drivers/media/video/cx88/cx88-i2c.c:102: warning: unused variable 'tun_setup'

Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>