usb: gadget: allow android gadget to bind/unbind several times

This is needed if we want to change the underlying udc, or insmod
the dynamically linked udc after android gadget is already up.

Change-Id: I71923764af85f4d50a2ebf7c7476cf0ef23fe819
Signed-off-by: Lena Salman <esalman@codeaurora.org>