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: 7b52db39

 ("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>