usb: gadget: uvc: fix changing interface name via configfs

usb: gadget: uvc: fix changing interface name via configfs

When setting the function name, it is always truncated by one char since
snprintf is always including the null-termination in the len parameter.
We use strscpy and fix the size setting to use len + 1 instead.

Fixes: 324e4f85070f ("usb: gadget: uvc: allow changing interface name via configfs")
Signed-off-by: Michael Grzeschik <m.grzeschik@pengutronix.de>
Link: https://lore.kernel.org/r/20220707115612.2760569-1-m.grzeschik@pengutronix.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>