USB: ci13xxx_udc: handle controllers with less than 16 EPs

The ci13xxx_udc driver checks the number of endpoints in the udc
controller, however some routines expect that the hardware has 16
bidirectional endpoints.

This patch improves the driver to work on controllers with less than
16 endpoints like the udc controller found on freescale's
mx23 and mx28.

Tested-by: Pavankumar Kondeti <pkondeti@codeaurora.org>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>