USB: gadget: diag: Check interface number before updating PID

Currently DIAG driver updates PID and serail number to DLOAD
without checking interface number during bind. This results in
updating PID and serial number to DLOAD in case of nonzero DIAG
composition as well. If crash happens in USB composition that
doesn't have DIAG as first interface, it requires user intervention
to install drivers in DLOAD mode.  Hence check interface number
and update PID and serail number only if DIAG is first interface.

CRs-Fixed: 472691
Change-Id: I33c2e670cbe7cc997164d152a5c52ad184772e36
Signed-off-by: Vijayavardhan Vennapusa <vvreddy@codeaurora.org>