[cpp][py] led.set_{on,off}_delay() only work before led.flash() is called

[ddemidov]

I have only checked this from Python API for now:

>>> from ev3dev import *
>>> led.red_right.trigger = 'timer'
>>> led.red_right.set_on_delay(200) # Both of these
>>> led.red_right.set_off_delay(200) # work as expected
>>> led.red_right.flash(200)
---------------------------------------------------------------------------
RuntimeError                              Traceback (most recent call last)
<ipython-input-7-2c6e48ea06cd> in <module>()
----> 1 led.red_right.flash(200)

RuntimeError: No such device
>>> led.red_right.set_on_delay(200)
---------------------------------------------------------------------------
RuntimeError                              Traceback (most recent call last)
<ipython-input-8-e18c136e252c> in <module>()
----> 1 led.red_right.set_on_delay(200)

RuntimeError: No such device

[ddemidov]

Probably related to ev3dev/ev3dev#225