Add framework for exclusive radio operations

Add framework for exclusive radio operations

The new radio work item concept can be used to request time for an
operation that requires exclusive radio control, e.g., a scan. Once the
radio is available, the registered callback function will be called.
radio_work_done() must be called once the exclusive radio operation has
been completed, so that the radio is freed for other operations. The
special case of deinit=1 is used to free the context data during
interface removal. That does not allow the callback function to start
the radio operation, i.e., it needs to free the allocated resources
and return.

Signed-hostap: Jouni Malinen <j@w1.fi>