RecordTimer.py: Reset first_try_prepare on repeating timers

See http://openpli.org/forums/topic/30676-problem-with-a-repeated-timers/

Initially 'first_try_prepare' set to True. So, if current channel isn't the
required one, then RecordTimerEntry.activate() switches to the required
channel. But this method also sets the 'first_try_prepare' variable to
False. So on subsequent recording events RecordTimerEntry.activate() can't
switch to the required channel.