wifi: ath: dfs_pattern_detector: Fix a memory initialization issue

If an error occurs and channel_detector_exit() is called, it relies on
entries of the 'detectors' array to be NULL.
Otherwise, it may access to un-initialized memory.

Fix it and initialize the memory, as what was done before the commit in
Fixes.

Fixes: a063b650 ("ath: dfs_pattern_detector: Avoid open coded arithmetic in memory allocation")
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/ad8c55b97ee4b330cb053ce2c448123c309cc91c.1695538105.git.christophe.jaillet@wanadoo.fr