usb: gadget: pxa27x: Remove duplicate function prototype

Functions udc_enable() and udc_disable() have a duplicated prototype.
Remove it.

Acked-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Petr Cvek <petr.cvek@tul.cz>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>