media: allegro: add field for number of buffers

When pushing the buffers for the intermediate and reference frames to
the MCU, the driver relied on the message size to calculate the number
of buffers. As it is not necessary anymore to keep the messages binary
compatible to the firmware, we can just explicitly write the number of
buffers into the message.

Signed-off-by: Michael Tretter <m.tretter@pengutronix.de>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>