media: pci: cx23885: replace BUG with error return

It was completely unnecessary to use BUG in buffer_prepare().
Just replace it with an error return. This also fixes a smatch warning:

drivers/media/pci/cx23885/cx23885-video.c:422 buffer_prepare() error: uninitialized symbol 'ret'.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>