USB: gadget: pxa25x compiler warning fix

Fix config-dependent compiler warning:

  CC      drivers/usb/gadget/pxa25x_udc.o
drivers/usb/gadget/pxa25x_udc.c: In function 'pxa25x_udc_irq':
drivers/usb/gadget/pxa25x_udc.c:1806: warning: array subscript is above array bounds

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>