[media] gspca: Use video_drvdata(file) instead of file->private_data

Prepare for control events: free up file->private_data by using
video_drvdata(file) to get to the gspca_dev struct.

[mchehab@redhat.com: fix a compile error: ‘file’ undeclared]
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>