usb: gadget: f_mtp: Handle OS descriptors properly

The mtp_ctrl_request function is responding to
every os descriptor sent by host,There by enumerating
the device with mtp composition. Once mtp is disabled,
need to clear the function instances on unbind.

Change-Id: I6679a1c1009df291a85ba8dcc34997d757c320b9
Signed-off-by: Pratham Pratap <prathampratap@codeaurora.org>