usbip: tools: add hint when no exported devices are found

usbip: tools: add hint when no exported devices are found

When refresh_exported_devices() finds no devices, it's helpful to
inform users about potential causes. This could be due to:

1. The usbip driver module is not loaded.
2. No devices have been exported yet.

Add an informational message to guide users when ndevs == 0.

Also update the condition in usbip_host_driver_open() and
usbip_device_driver_open() to check both ret and ndevs == 0,
and change err() to info().

Message visibility by scenario:
- usbipd (console mode): Show on console/serial, this allows instant
  visibility for debugging.
- usbipd -D (daemon mode): Message logged to syslog, can keep logs for
  later traceability in production. Also can use "journalctl -f" to
  trace on console.

Suggested-by: Shuah Khan <skhan@linuxfoundation.org>
Signed-off-by: Zongmin Zhou <zhouzongmin@kylinos.cn>
Reviewed-by: Shuah Khan <skhan@linuxfoundation.org>
Link: https://patch.msgid.link/20260402083204.53179-1-min_halo@163.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>