[media] em28xx: refactor get_next_buf() and use it for vbi data, too

get_next_buf() and vbi_get_next_buf() do exactly the same just with a
different dma queue and buffer. Saving the new buffer pointer back to the
device struct in em28xx_urb_data_copy() instead of doing this from inside
these functions makes it possible to get rid of one of them.
Also refactor the function parameters and return type:
- pass a pointer to struct em28xx as parameter (instead of obtaining the
  pointer from the dma queue pointer with the container_of macro) like we do
  it in all other functions
- instead of using a pointer-pointer, return the pointer to the new buffer
  as return value of the function

Signed-off-by: Frank Schäfer <fschaefer.oss@googlemail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>