Bluetooth: prevent dup DISCOVERY_STOP's pending

It is possible to redundently request Discovery
operations.  This fix cleans up some stored state to
prevent MGMT event overload when discovery finally completes.

Change-Id: I2ffb5c0749df95e40cd39e8c64c87a62a93fb896
CRs-fixed: 329657
Signed-off-by: Brian Gix <bgix@codeaurora.org>