drm/amd/powerplay: return errno code to caller when error occur

return errno code to caller when error occur, and meanwhile
remove gcc '-Wunused-but-set-variable' warning.

drivers/gpu/drm/amd/amdgpu/../powerplay/smumgr/vegam_smumgr.c: In function vegam_populate_smc_boot_level:
drivers/gpu/drm/amd/amdgpu/../powerplay/smumgr/vegam_smumgr.c:1364:6: warning: variable result set but not used [-Wunused-but-set-variable]

Reviewed-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: Chen Wandun <chenwandun@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>