usb: gadget: udc: udc-xilinx: Add identifier to read_fn function arg

Add an identifier in the read_fn function declaration because based on
commit ca0d8929 ("checkpatch: add warning for unnamed function
definition arguments") it is the preferred coding style even C standard
allows both formats.

Signed-off-by: Piyush Mehta <piyush.mehta@amd.com>
Message-ID: <20230607072959.2334046-1-piyush.mehta@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>