drm/amdgpu: Remove unneeded variable 'ret' in navi10_ih.c

Fixes coccicheck warning:

drivers/gpu/drm/amd/amdgpu/navi10_ih.c:113:5-8: Unneeded variable: "ret". Return "0" on line 182

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Ma Feng <mafeng.ma@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>