nimble/eatt: Handle MTU request over enhanced bearer

Handle ATT MTU request over enhanced bearer.
This request shall return Request Not
Supported error (0x06)

Author: szymon-czapracki

nimble/host/src/ble_att.c

@@ -639,6 +639,7 @@ bool
 ble_eatt_supported_req(uint8_t opcode)
 {
     switch (opcode) {
+    case BLE_ATT_OP_MTU_REQ:
     case BLE_ATT_OP_WRITE_CMD:
     case BLE_ATT_OP_FIND_INFO_REQ:
     case BLE_ATT_OP_FIND_TYPE_VALUE_REQ:

nimble/host/src/ble_eatt.c

@@ -269,6 +269,9 @@ ble_eatt_l2cap_event_fn(struct ble_l2cap_event *event, void *arg)
         if (ble_eatt_supported_rsp(opcode)) {
             ble_npl_eventq_put(ble_hs_evq_get(), &eatt->wakeup_ev);
         } else if (!ble_eatt_supported_req(opcode)) {
+            if (opcode == BLE_ATT_OP_MTU_REQ) {
+                return BLE_HS_ENOTSUP;
+            }
             /* If an ATT PDU is supported on any ATT bearer, then it shall be
              * supported on all supported ATT bearers with the following
              * exceptions: