char/serial: Fix emptyness check This was guarding against a full fifo rather than an empty fifo when popping. Fix. Signed-off-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com> Reviewed-by: Martin Kletzander <mkletzan@redhat.com> Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com> Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>