drm/amd/powerplay: fix a signedness bugs

Smatch complains about a signedness bug here:

        vega10_hwmgr.c:4202 vega10_force_clock_level()
        warn: always true condition '(i >= 0) => (0-u32max >= 0)'

Fixes: 7b52db39a4c2 ("drm/amd/powerplay: fix bug sclk/mclk
                     level can't be set on vega10.")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Eric Huang <JinHuiEric.Huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/powerplay/hwmgr/vega10_hwmgr.c

@@ -4186,7 +4186,7 @@ static int vega10_force_clock_level(struct pp_hwmgr *hwmgr,
 		enum pp_clock_type type, uint32_t mask)
 {
 	struct vega10_hwmgr *data = (struct vega10_hwmgr *)(hwmgr->backend);
-	uint32_t i;
+	int i;
 
 	if (hwmgr->dpm_level != AMD_DPM_FORCED_LEVEL_MANUAL)
 		return -EINVAL;