USB: gadget: composite: Don't enable endpoints if already enabled

As part of USBCV complaince, host sends set interface request
after set configuration request. This will reenable the endpoints
which are already enabled. This could cause WARN_ON message by
DCD. Hence check whether config is active or not and allow to call
function driver's set_alt if alternate setting is non zero.

CRs-Fixed: 486281
Change-Id: Id90c50f364b446329a38cd3b7389e4d0638d9023
Signed-off-by: Vijayavardhan Vennapusa <vvreddy@codeaurora.org>