post_proc: fix reverb processing issue when preset is set to NONE

Reverb effects are getting applied even after setting reverb preset
to REVERB_PRESET_NONE.
Preset NONE is used by client and is supposed to be equivalent to
disabled state, even though this preset is not supported in DSP.
Fix this issue by avoiding enable command from post processing HAL
to DSP when preset NONE is in use.

CRs-Fixed: 868041
Change-Id: Iad626a7400246e80a97a926234cfb7756ec6083e