drm/amdgpu/atomfirmware: silence UBSAN warning

This is a variable sized array.

Link: https://lists.freedesktop.org/archives/amd-gfx/2024-June/110420.html
Tested-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org