media: allegro: rework read/write to mailbox

Rework the functions that read and write the SRAM that is used to
communicate with the MCU.

As the functions will not operate on structs but on prepared binary
buffers, make the buffer stride more explicit. Also, avoid any uses of
struct mcu_msg_header to analyze messages in memory, because the header
will be made independent of the binary representation in the mailbox.
Instead explicitly access the mail size field in the mailbox.

As at it, further reduce the dependency between the mailboxes and struct
allegro_dev.

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>