net: dsa: mxl862xx: avoid unaligned 16-bit access in api_wrap

net: dsa: mxl862xx: avoid unaligned 16-bit access in api_wrap

The MXL862XX_API_* macros pass the address of a stack-allocated, __packed
firmware-ABI struct to mxl862xx_api_wrap() as a void *. The struct has an
alignment of 1, so the compiler is free to place it at an odd address.

mxl862xx_api_wrap() reinterprets that buffer as a __le16 * and accesses it
with data[i], for which the compiler assumes the natural 2-byte alignment
of __le16 and emits aligned 16-bit loads/stores (e.g. lhu/sh on MIPS).
When the buffer lands on an odd address these fault on architectures that
do not support unaligned access, such as MIPS32.

-Waddress-of-packed-member does not catch this: the packed origin is
laundered through the void * parameter, so the cast inside api_wrap looks
alignment-safe to the compiler and no warning is emitted.

Use get_unaligned_le16()/put_unaligned_le16() for the three 16-bit word
accesses. The byte accesses (*(u8 *)&data[i], crc16()) are already safe
and are left unchanged.

Link: https://sashiko.dev/#/patchset/cover.1781319534.git.daniel%40makrotopia.org?part=4
Fixes: 23794bec1cb6 ("net: dsa: add basic initial driver for MxL862xx switches")
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Link: https://patch.msgid.link/599327521db465a534d277de53ab9b6cac01928b.1781702256.git.daniel@makrotopia.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>