V4L/DVB: s2255drv: video_device_alloc call not checked fix

call to video_device_alloc was not being checked in probe function.
code simplified and uses video_device inside device structure.

Signed-off-by: Dean Anderson <dean@sensoray.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>