policy_hal: avoid VOIP_RX flag is removed for voip call

When offload output starts with FLAG_DIRECT, then setup a
voip call, VOIP_RX flag will be overwritten to FLAG_NONE.
This causes voip usecase isn't set for voip call.

Fix by adding VOIP_RX flag check to avoid settting 
FLAG_NONE for voip call.

Change-Id: Iee89d53d7529e6235b31903376b30e3a23657e3f