usb: gadget: f_rndis: Protect RNDIS options with mutex

usb: gadget: f_rndis: Protect RNDIS options with mutex

The class/subclass/protocol options are suspectible to race conditions
as they can be accessed concurrently through configfs.

Use existing mutex to protect these options. This issue was identified
during code inspection.

Fixes: 73517cf49bd4 ("usb: gadget: add RNDIS configfs options for class/subclass/protocol")
Cc: stable@vger.kernel.org
Signed-off-by: Kuen-Han Tsai <khtsai@google.com>
Link: https://patch.msgid.link/20260320-usb-net-lifecycle-v1-2-4886b578161b@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>