V4L/DVB (13180): gspca-w9968cf: Don't add an extra packet to the buffer after the EOF

gspca-w9968cf: Don't add an extra packet to the buffer after the EOF,
this makes the frame data size bigger then it should be which makes
gstreamer discard it.

This patch also fixes an unrelated compiler warning.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>