usb: gadget: udc: at91: switch to using fwnode_gpiod_get_index()

I would like to stop exporting OF-specific gpiod_get_from_of_node()
so that gpiolib can be cleaned a bit, so let's switch to the generic
fwnode property API.

Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

Link: https://lore.kernel.org/r/20220903-gpiod_get_from_of_node-remove-v1-5-b29adfb27a6c@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>