usb: gadget: configfs: Synchronizing bind/unbind using mutex lock

Synchronization is required because bind/unbind can happen in two
different contexts. For example, function unbind due to
unregister_gadget_item can race with bind which is initiated from
sysfs.

Change-Id: Iec93a9d4cb2a10b833cbba0d5256b64161147de2
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>