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.

Signed-off-by: Badhri Jagan Sridharan <badhri@google.com>
Change-Id: I248cb7175d0dd9a51c18053dd39475d8b3284f6d