usb: misc: usbtest: send ISO packets for g_zero

since commit b4036ccd (usb: gadget: add
isochronous support to gadget zero), g_zero
has learned about isochronous transfers, which
allows us to use usbtest.ko to exercise
isochronous pipes.

All we need to do to enable that functionality
on usbtest.ko, is set the "iso" to 1 on
struct usbtest_info

Signed-off-by: Boyan Nedeltchev <boyan@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>