media: stk1160: move transfer_buffer and urb to same struct 'stk1160_urb'

Instead of having two separated arrays, one for the urbs and
one for their buffers, have one array of a struct containing both.
In addition, the array is just 16 pointers, no need to dynamically
allocate it.

Signed-off-by: Dafna Hirschfeld <dafna.hirschfeld@collabora.com>
Reviewed-by: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>