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. Also, is_ptp flag is set
incorrectly as true for mtp instance which would
pass wrong os descriptor for mtp and ptp instance.
Allow mtp driver to send os descriptor by populating
function pointer in function instance and correctly
set the is_ptp flag to pass valid os descriptors.

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