usb: musb: ux500: move channel number knowledge into the driver

For all ux500 based platforms the maximum number of end-points are used.
Move this knowledge into the driver so we can relinquish the burden from
platform data. This also removes quite a bit of complexity from the driver
and will aid us when we come to enable the driver for Device Tree.

Cc: linux-usb@vger.kernel.org
Acked-by: Felipe Balbi <balbi@ti.com>
Acked-by: Fabio Baltieri <fabio.baltieri@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>