usbip: tools: add usage of device mode in usbip_list.c

The option '-d/--device' was implemented in 'usbip list' but not
shown in usage. Hence this commit adds this option to usage.

Acked-by: Shuah Khan <[e-mail]>
Signed-off-by: Hongren Zheng <[e-mail]>
Link: https://lore.kernel.org/r/YFrwq75Uyef3c9gz@Sun
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

tools/usb/usbip/src/usbip_list.c

···
 	"usbip list [-p|--parsable] <args>\n"
 	"    -p, --parsable         Parsable list format\n"
 	"    -r, --remote=<host>    List the exportable USB devices on <host>\n"
-	"    -l, --local            List the local USB devices\n";
+	"    -l, --local            List the local USB devices\n"
+	"    -d, --device           List the local USB gadgets bound to usbip-vudc\n";
 
 void usbip_list_usage(void)
 {