media: usbtv: Use managed buffer allocation

Clean up the driver with the new managed buffer allocation API.
The hw_params and hw_free callbacks became superfluous and dropped.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>