V4L/DVB (7658): dvb-core: Fix DMX_SET_BUFFER_SIZE in case the buffer shrinks

This patch fixes the bug in DMX_SET_BUFFER_SIZE for the demux.
Basically it resets read and write pointers to 0 in case they are
beyond the new size of the buffer.

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>