drm/amdgpu: Fix signedness bug in sdma_v4_0_process_trap_irq()

The "instance" variable needs to be signed for the error handling to work.

Fixes: 8b2faf1a ("drm/amdgpu: add error handle to avoid out-of-bounds")
Reviewed-by: Bob Zhou <bob.zhou@amd.com>
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>