usb: gadget: Invalidate interface id upon adding function to config

Commit e9df68ade ("usb: gadget: f_mbim: Queue notification request
upon function resume") added variable to store interface id in
usb_interface_id(). This interface id is used to send function wake
device notification. Functions having control interface and data
interface calls usb_interface_id() twice causing invalid interface
index stored in the variable. This is causing remote wakeup failure.
Fix this issue by initializing variable to -EINVAL and set it to
interface index only when it is invalidated.

CRs-Fixed: 810737
Change-Id: I41bce0a71ce0b8e92f3736d4e292132ce16de021
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>
Signed-off-by: Azhar Shaikh <azhars@codeaurora.org>