firmware: thead: Fix buffer overflow and use standard endian macros

firmware: thead: Fix buffer overflow and use standard endian macros

Addresses two issues in the TH1520 AON firmware protocol driver:

1. Fix a potential buffer overflow where the code used unsafe pointer
   arithmetic to access the 'mode' field through the 'resource' pointer
   with an offset. This was flagged by Smatch static checker as:
   "buffer overflow 'data' 2 <= 3"

2. Replace custom RPC_SET_BE* and RPC_GET_BE* macros with standard
   kernel endianness conversion macros (cpu_to_be16, etc.) for better
   portability and maintainability.

The functionality was re-tested with the GPU power-up sequence,
confirming the GPU powers up correctly and the driver probes
successfully.

[   12.702370] powervr ffef400000.gpu: [drm] loaded firmware
powervr/rogue_36.52.104.182_v1.fw
[   12.711043] powervr ffef400000.gpu: [drm] FW version v1.0 (build
6645434 OS)
[   12.719787] [drm] Initialized powervr 1.0.0 for ffef400000.gpu on
minor 0

Fixes: e4b3cbd840e5 ("firmware: thead: Add AON firmware protocol driver")
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Closes: https://lore.kernel.org/all/17a0ccce-060b-4b9d-a3c4-8d5d5823b1c9@stanley.mountain/
Signed-off-by: Michal Wilczynski <m.wilczynski@samsung.com>
Reviewed-by: Dan Carpenter <dan.carpenter@linaro.org>
Acked-by: Drew Fustini <fustini@kernel.org>
Cc: stable@vger.kernel.org
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>