wifi: ath: dfs_pattern_detector: Use flex array to simplify code

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@wanadoo.fr>
Reviewed-by: Jeff Johnson <quic_jjohnson@quicinc.com>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/1920cc38db2e570633e13b37d50852f3202a7270.1695538105.git.christophe.jaillet@wanadoo.fr