drm/amd/powerplay: Remove unneeded variable 'ret' in smu7_hwmgr.c

Fixes coccicheck warning:

drivers/gpu/drm/amd/powerplay/hwmgr/smu7_hwmgr.c:5188:5-8: Unneeded variable: "ret". Return "0" on line 5196

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: zhengbin <zhengbin13@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>