usb: gadget: gr_udc: Use of_property_read_u32_index to access arrays

Use an appropriate accessor function for property arrays to make the code nicer
and make the code correct if it would ever run on little endian architectures.
Suggested by Mark Rutland.

Signed-off-by: Andreas Larsson <andreas@gaisler.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>