nimble/eatt: Handle MTU request over enhanced bearer

Handle ATT MTU request over enhanced bearer.
This request shall return Request Not
Supported error (0x06)
This is required for GATT/CL/GAC/BV-01-C test.

Author: szymon-czapracki

nimble/host/src/ble_att.c

@@ -639,6 +639,10 @@ bool
 ble_eatt_supported_req(uint8_t opcode)
 {
     switch (opcode) {
+    /* EATT does not support MTU request,
+     * but we must handle this request with proper error response.
+     */
+    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_att_svr.c

@@ -747,7 +747,7 @@ ble_att_svr_rx_mtu(uint16_t conn_handle, uint16_t cid, struct os_mbuf **rxom)
     mtu = 0;
 
     if (cid != BLE_L2CAP_CID_ATT) {
-        /*TODO */
+        /* This operation is not supported over enhanced ATT bearer */
         return BLE_HS_ENOTSUP;
     }