extmod/modbluetooth: Simplify how BLE IRQ callback is scheduled.
extmod/modbluetooth: Simplify how BLE IRQ callback is scheduled.

Instead of enqueue_irq() inspecting the ringbuf to decide whether to
schedule the IRQ callback (if ringbuf is empty), maintain a flag that knows
if the callback is on the schedule queue or not.  This saves about 150
bytes of code (for stm32 builds), and simplifies all uses of enqueue_irq()
and schedule_ringbuf().

pfalcon/micropython,pfalcon/micropython,pfalcon/micropython,pfalcon/micropython,pfalcon/micropython