usb: dwc3: debugfs: Add LPM check for ep_info_register

Prevent unclocked access if this debugfs parameter is
attempted to be read when USB controller is powered off.

Change-Id: I5a563ea75807d95f0d6d6c5b037db48681d89625
Signed-off-by: Jack Pham <jackp@codeaurora.org>