drm/amdgpu: Clean up errors in mxgpu_vi.c

Fix the following errors reported by checkpatch:

ERROR: spaces required around that '-=' (ctx:WxV)
Signed-off-by: Ran Sun <sunran001@208suo.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdgpu/mxgpu_vi.c

@@ -334,7 +334,7 @@ static void xgpu_vi_mailbox_send_ack(struct amdgpu_device *adev)
 			break;
 		}
 		mdelay(1);
-		timeout -=1;
+		timeout -= 1;
 
 		reg = RREG32_NO_KIQ(mmMAILBOX_CONTROL);
 	}