usb: usbtest: fix checkpatch warning as sizeof code style

Script checkpatch.pl always complains incorrect code style like below:

WARNING: sizeof *udev->bos->desc should be sizeof(*udev->bos->desc)

This patch fixes the warning for usbtest module.

Signed-off-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>