drm/amd/pm: Return -EOPNOTSUPP when can't read power limit

drm/amd/pm: Return -EOPNOTSUPP when can't read power limit

So that hwmon_attributes_visible() will see that the power2_cap
attributes should not be visible on GPUs that don't support
the get_power_limit() function.

This fixes an error when running the "sensors" command on SI.

Fixes: 12c958d1db36 ("drm/amd/pm: Expose ppt1 limit for gc_v9_5_0")
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>