drm/amd/powerplay: vega20: fix uninitialized variable use

If smu_get_current_rpm() fails, we can't use the output,
as that may be uninitialized:

drivers/gpu/drm/amd/amdgpu/../powerplay/vega20_ppt.c:3023:8: error: variable 'current_rpm' is used uninitialized whenever '?:' condition is false [-Werror,-Wsometimes-uninitialized]
        ret = smu_get_current_rpm(smu, &current_rpm);
              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/gpu/drm/amd/amdgpu/../powerplay/inc/amdgpu_smu.h:735:3: note: expanded from macro 'smu_get_current_rpm'
        ((smu)->funcs->get_current_rpm ? (smu)->funcs->get_current_rpm((smu), (speed)) : 0)
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/gpu/drm/amd/amdgpu/../powerplay/vega20_ppt.c:3024:12: note: uninitialized use occurs here
        percent = current_rpm * 100 / pptable->FanMaximumRpm;
                  ^~~~~~~~~~~
drivers/gpu/drm/amd/amdgpu/../powerplay/vega20_ppt.c:3023:8: note: remove the '?:' if its condition is always true
        ret = smu_get_current_rpm(smu, &current_rpm);
              ^
drivers/gpu/drm/amd/amdgpu/../powerplay/inc/amdgpu_smu.h:735:3: note: expanded from macro 'smu_get_current_rpm'
        ((smu)->funcs->get_current_rpm ? (smu)->funcs->get_current_rpm((smu), (speed)) : 0)
         ^
drivers/gpu/drm/amd/amdgpu/../powerplay/vega20_ppt.c:3020:22: note: initialize the variable 'current_rpm' to silence this warning
        uint32_t current_rpm;

Propagate the error code in that case.

Fixes: ee0db820 ("drm/amd/powerplay: move PPTable_t uses into asic level")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>