dwc3: gadget: Update ep_num with endpoint structure

dwc3_gadget_resize_tx_fifos() and usb_ep_autoconfig_by_name() APIs
use ep_num. Hence update ep_num with endpoint structure when
initializing USB endpoints.

Change-Id: I59782e4e66aceeab6274485800a0d5175fd2cae4
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>