V4L/DVB (9676): em28xx: fix a regression caused by 22c90ec6a5e07173ee670dc2ca75e0df0a7772c0

If removing and reiserting the driver on some devices, tuner type will
be unset at the second time. This patch fixes this issue.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>