nimble/ll: Fix scanner init on HCI Reset
nimble/ll: Fix scanner init on HCI Reset

mempool shouldn't be initialized on HCI Reset as it may affect other
pools. Instead initialize it only on init and cleanup on HCI Reset.

apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble