usb: dwc3: gadget: warn about endpoint already enabled before changing ep name

In case some gadget driver tries to enable an endpoint
which is already enabled, we print a nice WARN so we
can track broken gadget drivers. The only problem is that
we're printing the WARN when we already changed endpoint's
name, which would result in endpoints named as:

ep1in-bulk-bulk-bulk-bulk-bulk-bulk-bulk

To prevent that, we will continue to print the WARN,
but do so before changing endpoint's name and return
early.

Signed-off-by: Felipe Balbi <balbi@ti.com>