media: allegro: don't pack MCU messages

The messages are now explicitly converted from the struct to the binary
representation used by the firmware. Therefore, there is no need to keep
the structs packed anymore.

Drop the attribute and avoid confusion why the mails are packed.

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>