media: allegro: drop length field from message header

The length of the message will be determined when the message is
encoded.  Writing the size of the struct into the message in the driver
won't be the actual length of the message that is send to the firmware.
Therefore, drop the length field from the message.

Since the header is the same for all response messages, it does not make
sense to read the header in each decoding function, but we can simply
decode it once before dispatching to the respective functions.

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>