usb_scan_devices: fix output with no devices

We should check the return of usb_new_device() so that if no USB device is
found, we print out the right message rather than always saying "new usb
device found".

Signed-off-by: Bryan Wu <bryan.wu@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Remy Bohmer <linux@bohmer.net>