USB: ci13xxx: Bail out from ep fifo_flush if USB in LPM

Function drivers might call ep fifo_flush in LPM. Bail
out with error message if so to avoid unclocked access
in case there is any pending response.

Change-Id: I99816a13b4492f73d2c210c2bbe46f27d0870700
Signed-off-by: Manu Gautam <mgautam@codeaurora.org>