usb: gadget: f_mtp: Handle mtp os desc properly

Currently mtp function driver is skipping function
setup call back by not populating function pointer
in function instance. Allow mtp driver to send OS
descriptor by populating function pointer.

Change-Id: Iec05a2f9309aba7d9ff81eca4ce683eff1590699
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>