usb: dwc3: Check USB LPM status before accessing registers

USB controller registers can be accessed when USB session
is not active or USB is in low power mode. This results
in unclocked register access. Prevent this by checking
the USB LPM status before accessing the registers.

Change-Id: I5a31bda1d88ac65b28afc2b545a44b4574b8548a
Signed-off-by: Elson Roy Serrao <eserrao@codeaurora.org>