usb: gadget: udc: fsl: make use of new usb_endpoint_maxp_mult()

We have introduced a helper to calculate multiplier
value from wMaxPacketSize. Start using it.

Cc: Li Yang <leoli@freescale.com>
Cc: <linuxppc-dev@lists.ozlabs.org>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>