usb: gadget: f_acm: Allow restarting ACM console after stopping it

usb: gadget: f_acm: Allow restarting ACM console after stopping it

When using IOMUX, the "usbacm" console can be added/removed dynamically
from the stdout/stderr/stdin environment variables to allow temporarily
starting other USB gadgets (e.g. Fastboot).

However, right now acm_stdio_stop() does not completely undo
acm_stdio_start(): The USB gadget is unregistered, but as long as dev->priv
stays set acm_stdio_start() will never register the USB gadget again.

Clear dev->priv after we detach to make sure a start operation after a stop
operation registers the gadget again.

Fixes: fc2b399ac03b ("usb: gadget: Add CDC ACM function")
Signed-off-by: Stephan Gerhold <stephan.gerhold@linaro.org>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@kernel.org>
Link: https://lore.kernel.org/r/20250407-acm-fixes-v1-2-e3dcb592d6d6@linaro.org
Signed-off-by: Mattijs Korpershoek <mkorpershoek@kernel.org>