ltq-vdsl-vr9-mei: avoid unnecessary usage of unsafe_memcpy

ltq-vdsl-vr9-mei: avoid unnecessary usage of unsafe_memcpy

Commit 7bc487c12eef ("kernel: ltq-vdsl-vr9-mei: fix warning about
field-spanning write") patched the driver to use unsafe_memcpy in
MEI_IoctlCmdMsgWrite.

However, this is not actually necessary. The assignment of the variable
"pDestPtr" can be modified so that the compiler knows about the correct
size. This way, the check in the fortified memcpy works correctly.

While at it, also adjust all places where similar code is used to copy
from a CMV_STD_MESSAGE_T struct.

Also mark all related structs as packed, because the code (and the
driver in general) seems to rely on that anyway.

Signed-off-by: Jan Hoffmann <jan@3e8.eu>
Link: https://patchwork.ozlabs.org/project/openwrt/patch/20250206225444.2521817-2-jan@3e8.eu/
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>