udc: lpc32xx: mark local function static

The kernel test robot reports two functions that should be marked
static:

>> drivers/usb/gadget/udc/lpc32xx_udc.c:1928:6: warning: no previous prototype for 'udc_send_in_zlp' [-Wmissing-prototypes]
    1928 | void udc_send_in_zlp(struct lpc32xx_udc *udc, struct lpc32xx_ep *ep)
>> drivers/usb/gadget/udc/lpc32xx_udc.c:1942:6: warning: no previous prototype for 'udc_handle_eps' [-Wmissing-prototypes]
    1942 | void udc_handle_eps(struct lpc32xx_udc *udc, struct lpc32xx_ep *ep)

This showed up after my commit 792e559e ("udc: lpc32xx: fix 64-bit
compiler warning") made it possible to build the driver on x86-64.

Fix the warning as suggested.

Reported-by: kernel test robot <lkp@intel.com>
Acked-by: Vladimir Zapolskiy <vz@mleia.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>