usb: gadget: Update MAC address for RNDIS

Without this change, the MAC address which is
delivered to the host is null.
This fix updates RNDIS MAC address with the correct
MAC address.

Change-Id: Idcefd2f6f1895cbd632d9360fbaf8ada16a0f591
Signed-off-by: Bar Weiner <bweiner@codeaurora.org>