ath: drop unnecessary list_empty

list_for_each_entry{_safe} is able to handle an empty list.
The only effect of avoiding the loop is not initializing the
index variable.
Drop list_empty tests in cases where these variables are not
used.

Note that list_for_each_entry{_safe} is defined in terms of
list_first_entry, which indicates that it should not be used on an
empty list.  But in list_for_each_entry{_safe}, the element obtained
by list_first_entry is not really accessed, only the address of its
list_head field is compared to the address of the list head, so the
list_first_entry is safe.

The semantic patch that makes this change for the list_for_each_entry
case is as follows: (http://coccinelle.lip6.fr/)

<smpl>
@@
expression x,e;
statement S;
identifier i;
@@

-if (!(list_empty(x)))
   list_for_each_entry(i,x,...) S
 ... when != i
? i = e
</smpl>
Signed-off-by: Julia Lawall <Julia.Lawall@inria.fr>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/1595761112-11003-8-git-send-email-Julia.Lawall@inria.fr

drivers/net/wireless/ath/dfs_pattern_detector.c

@@ -253,7 +253,6 @@ channel_detector_get(struct dfs_pattern_detector *dpd, u16 freq)
 static void dpd_reset(struct dfs_pattern_detector *dpd)
 {
 	struct channel_detector *cd;
-	if (!list_empty(&dpd->channel_detectors))
 	list_for_each_entry(cd, &dpd->channel_detectors, head)
 		channel_detector_reset(dpd, cd);
 
@@ -261,7 +260,6 @@ static void dpd_reset(struct dfs_pattern_detector *dpd)
 static void dpd_exit(struct dfs_pattern_detector *dpd)
 {
 	struct channel_detector *cd, *cd0;
-	if (!list_empty(&dpd->channel_detectors))
 	list_for_each_entry_safe(cd, cd0, &dpd->channel_detectors, head)
 		channel_detector_exit(dpd, cd);
 	kfree(dpd);
@@ -331,7 +329,6 @@ static bool dpd_set_domain(struct dfs_pattern_detector *dpd,
 		return false;
 
 	/* delete all channel detectors for previous DFS domain */
-	if (!list_empty(&dpd->channel_detectors))
 	list_for_each_entry_safe(cd, cd0, &dpd->channel_detectors, head)
 		channel_detector_exit(dpd, cd);
 	dpd->radar_spec = rt->radar_types;