usb: gadget: Enable USB2 LPM Support

commit 7e115fdb ("usb: gadget: Disable USB2 LPM Support")
disables USB2 LPM because device fails to initiate L1 remote
wakeup. Commit 6f1c8202 ("usb: dwc3: Add support for
device L1 exit") fixes the issue for non-ep0 endpoints by
enabling HW exit from L1. Commit 589759f2 ("usb: dwc3: ep0:
NAK L1 initiated between SETUP and STATUS stage") NAKs L1 during
control transfers. Hence allow device to report bcdUSB as 2.1 in
high speed mode. This also passes USB2.0 CV TD 9.7 and USB3.0 CV
TD 9.23.

Change-Id: I77d6eb7f8d977d35fd1a5c61d1102a702734e6d9
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>