V4L/DVB (13153): uvcvideo: Fix uvc_alloc_urb_buffers()

This patch sets stream->urb_size as psize*npackets
before calling uvc_alloc_urb_buffers, which may fix
a possible failure of usb_buffer_free in case usb_buffer_alloc
returns NULL. The patch is based on the ideas below:

1,If usb_buffer_alloc can't allocate a buffer sucessfully,
uvc_free_urb_buffers will be called to free the allocated
buffers, and stream->urb_size is required to be passed to
usb_buffer_free;

2,uvc_free_urb_buffers can reset stream->urb_size.

This patch is against linux-v2.6.31-next-20090926.

Signed-off-by: Ming Lei <tom.leiming@gmail.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>