drm/amdgpu/atomfirmware: Silence UBSAN warning

Per the comments, these are variable sized arrays.

Closes: https://gitlab.freedesktop.org/drm/amd/-/issues/3613


Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit 81f7804b)
Cc: stable@vger.kernel.org