nimble/eatt: Handle MTU request over enhanced bearer

Handle ATT MTU request over enhanced bearer.
This request shall return Request Not
Supported error (0x06)
Now all unknown opcodes will force nimble eatt
to return unsupported error.

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

@@ -277,8 +277,7 @@ ble_eatt_l2cap_event_fn(struct ble_l2cap_event *event, void *arg)
              *  • The Signed Write Without Response sub-procedure shall only be
              *  supported on the LE Fixed Channel Unenhanced ATT bearer.
              */
-            ble_l2cap_disconnect(eatt->chan);
-            return BLE_HS_EREJECT;
+            return BLE_HS_ENOTSUP;
         }
 
         assert (!ble_gap_conn_find(event->receive.conn_handle, &desc));