drm/amd/pm: fix error handling

clang static analysis reports this error
amdgpu_smu.c:2289:9: warning: Called function pointer
  is null (null dereference)
        return smu->ppt_funcs->emit_clk_levels(
               ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

There is a logic error in the earlier check of
emit_clk_levels.  The error value is set to
the ret variable but ret is never used.  Return
directly and remove the unneeded ret variable.

Fixes: 5d64f9bb ("amdgpu/pm: Implement new API function "emit" that accepts buffer base and write offset")
Reviewed-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: Tom Rix <trix@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>