usb: misc: usbtest: allocate size of urb array according to user parameter

Allocate the size of urb pointer array according to testusb's
parameter sglen, and limits the length of sglen as MAX_SGLEN
(128 currently).

Acked-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Peter Chen <peter.chen@freescale.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>