media: staging: rkisp1: cap: protect buf.curr and buf.next with buf.lock

The spinlock buf.lock should protect access to the buffers.
This includes the buffers in buf.queue and also buf.curr and
buf.next. The function 'rkisp1_set_next_buf' access buf.next
therefore it should also be protected with the lock.

Signed-off-by: Dafna Hirschfeld <dafna.hirschfeld@collabora.com>
Acked-by: Helen Koike <helen.koike@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>