usb: gadget: f_sourcesink: format data pattern according to max packet size

Since the host and gadget can't agree with transfer length before
each transfer, but they agree with max packet size for each
endpoint, we use max packet size to format data pattern.

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