usb: usbtest: bmAttributes would better be masked

When transfer type is isochronous, the other bits (bits 5..2) of
bmAttributes in endpoint descriptor might not be set zero. So it's better
to use usb_endpoint_type routine to mask bmAttributes with
USB_ENDPOINT_XFERTYPE_MASK to judge the transfter type later.

Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>