[media] media: blackfin: bfin_capture: set vb2 buffer field

set the vb2 buffer field in buf_prepare() callback,
alongside drop local variable buf as we already have
a pointer to vb2 buffer.

Signed-off-by: Lad, Prabhakar <prabhakar.csengg@gmail.com>
Tested-by: Scott Jiang <scott.jiang.linux@gmail.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>