drm/amd/pm/swsmu: Avoid using structure_size uninitialized in smu_cmn_init_soft_gpu_m...

drm/amd/pm/swsmu: Avoid using structure_size uninitialized in smu_cmn_init_soft_gpu_metrics

Clang warns:

drivers/gpu/drm/amd/amdgpu/../pm/swsmu/smu_cmn.c:764:2: warning:
variable 'structure_size' is used uninitialized whenever switch default
is taken [-Wsometimes-uninitialized]
        default:
        ^~~~~~~
drivers/gpu/drm/amd/amdgpu/../pm/swsmu/smu_cmn.c:770:23: note:
uninitialized use occurs here
        memset(header, 0xFF, structure_size);
                             ^~~~~~~~~~~~~~
drivers/gpu/drm/amd/amdgpu/../pm/swsmu/smu_cmn.c:753:25: note:
initialize the variable 'structure_size' to silence this warning
        uint16_t structure_size;
                               ^
                                = 0
1 warning generated.

Return in the default case, as the size of the header will not be known.

Fixes: de4b7cd8cb87 ("drm/amd/pm/swsmu: unify the init soft gpu metrics function")
Link: https://github.com/ClangBuiltLinux/linux/issues/1304
Reviewed-by: Kevin Wang <kevin1.wang@amd.com>
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>