usb: f_mtp: Don't handle OS descriptors from MTP driver

Currently MTP driver is handling OS descriptors which
causes CTS test failures(android.adb.cts.AdbHostTest) if
MTP is in the default composition.

Fix this by skipping the handling of OS descriptors in
MTP driver and let composite driver take care of it for
the whole configuration.

Change-Id: I77902108c3a550997dfd56b9b3127900c724f066
Signed-off-by: Pratham Pratap <prathampratap@codeaurora.org>