Bluetooth: host: adv: revert 39cb574 to fix spurious error log

Commit 39cb574 changed the log level from LOG_DBG to LOG_ERR in
bt_le_adv_resume(). This causes the error log
"No valid legacy adv to resume" to appear during normal connection
establishment when using bt_le_ext_adv_start(), even though the
system is functioning correctly.

Revert the change to restore the original LOG_DBG level.

Fixes: #94954

Signed-off-by: Samuel Tsai <stsai@atmosic.com>
(cherry picked from commit 07dc0ad40d37907fb37bffa025226606b6232416)
diff --git a/subsys/bluetooth/host/adv.c b/subsys/bluetooth/host/adv.c
index 9fb69c6..f7ccdc6 100644
--- a/subsys/bluetooth/host/adv.c
+++ b/subsys/bluetooth/host/adv.c
@@ -1529,7 +1529,7 @@
 	int err;
 
 	if (!adv) {
-		LOG_ERR("No valid legacy adv to resume");
+		LOG_DBG("No valid legacy adv to resume");
 		return;
 	}