usb: host: xhci: make use of new usb_endpoint_maxp_mult()

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

Cc: Mathias Nyman <mathias.nyman@intel.com>
Cc: <linux-usb@vger.kernel.org>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>