V4L/DVB (7659): dvb-core: Implement DMX_SET_BUFFER_SIZE for dvr

Implementation of DMX_SET_BUFFER_SIZE for dvr.
Synchronization of the code of DMX_SET_BUFFER_SIZE for demux and dvr.

Signed-off-by: Andrea Odetti <mariofutire@gmail.com>
Signed-off-by: Oliver Endriss <o.endriss@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>