ANDROID: usb: gadget: Do not disconnect unregistered dev

configfs_composite_unbind sets the gadget data
to null. Therefore, add check in disconnect
function to make sure that cdev is not NULL.
Prints a WARN message if the driver tries to
redundantly disconnect a gadget.

Change-Id: Iad25f7a1e785ed336d34259c147efdcfc0840fae
Signed-off-by: Badhri Jagan Sridharan <badhri@google.com>
Git-commit: da17b0d6e3031d54deb05c08c7bf1616da43f7a6
Git-Repo: https://android.googlesource.com/kernel/common/


Signed-off-by: Pratham Pratap <prathampratap@codeaurora.org>