usb: abort due to `Packet_stream_base::Transport_dataspace_too_small`

[nfeske]

While using the USB API directly, the USB host driver aborted with the following message:

[usb] Error: Uncaught exception of type 'Genode::Packet_stream_base::Transport_dataspace_too_small'
[usb] Warning: abort called - thread: ep

The problem was triggered by creating a Usb::Interface instance with the size parameter set to 64 (I mistook the argument as URB size instead of the dataspace size for the communication buffer). May we sanitize the argument to a sensible lower bound that always satisfies the packet-stream preconditions?