bluetooth: hogp: Fix bt_hogp_rep_unsubscribe issue

The bt_hogp_rep_unsubscribe did not reset the rep->notify_cb,
while bt_hogp_rep_subscribe returned an error if
notify_cb was not null. Thus, it would be impossible
to subscribe again to the report notification after
unsubscribing.

This commit fixes this.

Signed-off-by: Artur Hadasz <artur.hadasz@nordicsemi.no>

Author: ahasztag

doc/nrf/releases_and_maturity/releases/release-notes-changelog.rst

@@ -477,7 +477,10 @@ Binary libraries
 Bluetooth libraries and services
 --------------------------------
 
-|no_changes_yet_note|
+* :ref:`hogp_readme` library:
+
+  * Fixed an issue where the :c:func:`bt_hogp_rep_unsubscribe` function did not clear the notification callback, which prevented the :c:func:`bt_hogp_rep_subscribe` function from succeeding after unsubscribing.
+    Also, the notification callback is no longer called with NULL data when the subscription is removed.
 
 Common Application Framework
 ----------------------------

include/bluetooth/services/hogp.h

@@ -407,6 +407,9 @@ int bt_hogp_rep_subscribe(struct bt_hogp *hogp,
 /**
  * @brief Remove the subscription for a selected report.
  *
+ * When the subscription removal is completed, the callback function
+ * will be called with data set to NULL.
+ *
  * @param hogp   HOGP object.
  * @param rep    Report object.
  *

subsys/bluetooth/services/hogp.c

@@ -1014,6 +1014,7 @@ static uint8_t rep_notify_process(struct bt_conn *conn,
 			       const void *data, uint16_t length)
 {
 	struct bt_hogp_rep_info *rep;
+	uint8_t err = 0;
 
 	rep = CONTAINER_OF(params,
 			   struct bt_hogp_rep_info,
@@ -1026,14 +1027,22 @@ static uint8_t rep_notify_process(struct bt_conn *conn,
 		LOG_WRN("Data size too big, truncating");
 		length = UINT8_MAX;
 	}
-	/* Zephyr uses the callback with data set to NULL to inform about the
-	 * subscription removal. Do not update the report size in that case.
-	 */
+
 	if (data != NULL) {
 		rep->size = (uint8_t)length;
 	}
 
-	return rep->notify_cb(rep->hogp, rep, 0, data);
+
+	err = rep->notify_cb(rep->hogp, rep, 0, data);
+
+	if (data == NULL) {
+		/* Zephyr uses the callback with data set to NULL to inform about the
+		 * subscription removal. Do not update the report size in that case.
+		 */
+		rep->notify_cb = NULL;
+	}
+
+	return err;
 }
 
 int bt_hogp_rep_subscribe(struct bt_hogp *hogp,