Use regular version of usbd_get_string
The RISC OS-specific version appears to be a leftover from an older revision of NetBSD that only seems to differ in the returned value and the fact that it terminates the buffer even on failure.
http://cvsweb.netbsd.org/bsdweb.cgi/src/sys/dev/usb/usb_subr.c.diff?r1=1.118&r2=1.119&only_with_tag=MAIN http://cvsweb.netbsd.org/bsdweb.cgi/src/sys/dev/usb/usbdi.c.diff?r1=1.104&r2=1.105&only_with_tag=MAIN