compress: compress_wait() must return error if timed out

The caller must be certain that a return of 0 really means
that compress is ready for more data, so when poll() returns
0 for a timeout we must report that as an error.

Signed-off-by: Richard Fitzgerald <rf@opensource.wolfsonmicro.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>