host/ble_att.c: add missing OP code in ble_att_is_request_op

Assert was hit when BLE_ATT_OP_NOTIFY_MULTI_REQ was received.

nimble/host/src/ble_att.c

@@ -649,6 +649,7 @@ ble_att_is_request_op(uint8_t opcode)
  case BLE_ATT_OP_EXEC_WRITE_REQ:
  case BLE_ATT_OP_INDICATE_REQ:
  case BLE_ATT_OP_READ_MULT_VAR_REQ:
+ case BLE_ATT_OP_NOTIFY_MULTI_REQ:
  return true;
  }
  return false;