USB: gadget: fix up const issue with struct usb_function_instance

In struct usb_function, the struct usb_function_instance pointer
variable "fi" is listed as const, but it is written to in numerous
places, making the const marking of it a total lie.  Fix this up by just
removing the const pointer attribute as this is modified in numerous
places.

Link: https://lore.kernel.org/r/2025052145-undress-puma-f7cf@gregkh


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>