qcacld-3.0: add NULL check before calling rps_enable cb

Add a NULL check before calling rps_enable callback
from ipa_set_rps_per_vdev and also add a check to avoid
disabling rps twice.

Change-Id: Ib63b2cda99aa332905df640ffa2d401c22845ad2
CRs-Fixed: 2876490