drm/amd/pm: optimize logic and remove unnecessary checks in smu v15.0.8

drm/amd/pm: optimize logic and remove unnecessary checks in smu v15.0.8

the following two sets of logic are clearly mutually exclusive in
smu_v15_0_8_set_soft_freq_limited_range.
remove unnecessary code logic to keep the code logic clear.

e.g:

if (smu_dpm->dpm_level != AMD_DPM_FORCED_LEVEL_MANUAL)
	return -EINVAL;

if (smu_dpm->dpm_level == AMD_DPM_FORCED_LEVEL_MANUAL) {
	...
}

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>