media: dvb_ringbuffer : Fix a bug in dvb_ringbuffer.c

media: dvb_ringbuffer : Fix a bug in dvb_ringbuffer.c

The function dvb_ringbuffer_pkt_next in
/linux-next/drviers/media/dvb-core/dvb_ringbuffer.c,
which searches the idx of the next valid packet in the ring
buffer of the ca->slot_info[slot].rx_buffer at
/linux-next/drivers/media/dvb-core/dvb_ca_en50221.c,
has the following problem.
In calculating the amounts of the consumed address of the ring
buffer, if the read address(rbuf->pread) of the ring buffer is
smaller than the idx, the amounts of the searched address
should be (idx - rbuf->pread),
whereas if the read address(rbuf->pread) of the ring buffer is
larger than the idx, the amounts of the consumed address should
be (idx - rbuf->pread + rbug->size). But there exists an
incorrect logic that the rbug-size was not properly added on
(idx - rbug->pread) in the later case. With this commit, we
fixed this bug.

Link: https://lore.kernel.org/linux-media/20220623103543.4138-1-yongsuyoo0215@gmail.com
Signed-off-by: Yongsu Yoo <yongsuyoo0215@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>