nrf51822/timer: fix critical bug

concerning the number of active timers
ll: forget to change an index -- to squash with prev commit
ll: implement timeslots management in connection intervals
ll: start and stop secondary operations

scan/init in connection state
ll: Handle timing issues for the first connection events
ll: never stop the interval timer if a connection is active
ll: forgot other conn_index..
nrf51822/timer: fix bug that caused invalid durations
when a timer is manually stopped, then fires (causes no IRQ), then is started
again, the compare event is still high for this channel and the timer will
fire on the next IRQ (caused by another channel).