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

@@ -457,7 +457,10 @@ 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
 ----------------------------

subsys/bluetooth/services/hogp.c

@@ -1026,11 +1026,13 @@ 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;
+	if (data == NULL) {
+		rep->notify_cb = NULL;
+		return BT_GATT_ITER_CONTINUE;
 	}
 
 	return rep->notify_cb(rep->hogp, rep, 0, data);