usb: gadget: udc: lpc32xx: allocate descriptor with GFP_ATOMIC

Gadget drivers may queue request in interrupt context. This would lead to
a descriptor allocation in that context. In that case we would hit
BUG_ON(in_interrupt()) in __get_vm_area_node.

Also remove the unnecessary cast.

Acked-by: Sylvain Lemieux <slemieux.tyco@gmail.com>
Tested-by: James Grant <jamesg@zaltys.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>