usb: gadget: make composite module options readonly only

This is a partly revert of 4fffd6e5 ("usb: gadget: composite: make
module parameters accessible at runtime").
It is not possible to change the VID or other property for a gadget
right now. This change has been made for Anrdoid gadget which has this
functionality in its copy of the file. This function is executed currently
only once and most caller in tree are __init.

Acked-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>