drm/radeon: fix variable type

When we switch to dma_resv_wait_timeout() the returned type changes as
well.

Signed-off-by: Christian König <christian.koenig@amd.com>
Fixes: 89aae41d ("drm/radeon: use dma_resv_wait_timeout() instead of manually waiting")
Bug: https://bugzilla.kernel.org/show_bug.cgi?id=215600


Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220221110503.2803-1-christian.koenig@amd.com