usb: usbtest: fix a type promotion bug

The "len" here is sometimes negative error codes from
usb_get_descriptor(), so we don't want to type promote them to unsigned
long.

This bug pre-dates the invention of git.

Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Link: https://lore.kernel.org/r/506f7935-2cba-41d9-ab5d-ddb6ad6320bd@moroto.mountain
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>