drm/amd/pm: fix the issue of size calculation error for smu 13.0.6

drm/amd/pm: fix the issue of size calculation error for smu 13.0.6

v1:
the driver should handle return value of smu_v13_0_6_printk_clk_levels()
to return the correct size for sysfs reads.

v2:
fix the issue of size calculation error in smu_v13_0_6_print_clks()

Fixes: cdfdec6f1608 ("drm/amd/pm: Avoid writing nulls into `pp_od_clk_voltage`")
Signed-off-by: Yang Wang <kevinyang.wang@amd.com>
Reviewed-by: Lijo Lazar <lijo.lazar@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>