At the time of the writing, the value of 'num_radar_types' is 7 or 9. So
on a 64 bits system, only 56 or 72 bytes are allocated for the
'detectors' array.
Turn it into a flex array, in order to simplify memory management and save
an indirection when the array is used.
Doing so, cd->detectors can't be NULL, and channel_detector_exit() can be
simplified as well.
Signed-off-by: Christophe JAILLET <christophe.jaillet@xxxxxxxxxx>