qcacmn: Add flush callback for scan module

Components are required to provide flush callback to scheduler which
gets invoked for all pending commands in scheduler queue when scheduler
thread is going down.
Added flush callback for scan so that command cleanup can be done.

Change-Id: I990a5e14db1498e13a2eeeaa605b44593b8442f1
CRs-Fixed: 2056542
4 files changed