spi: imx: replace schedule() with cond_resched()

It's more rational that just do the schedule when necessary
other than do it every time. Thus, it's better to replace
schedule() with cond_resched() in fsl_lpspi_txfifo_empty(),
which contributes to saving cpu time.
Signed-off-by: Gao Pan <pandy.gao@nxp.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-fsl-lpspi.c

@@ -176,7 +176,7 @@ static int fsl_lpspi_txfifo_empty(struct fsl_lpspi_data *fsl_lpspi)
 			dev_dbg(fsl_lpspi->dev, "txfifo empty timeout\n");
 			return -ETIMEDOUT;
 		}
-		schedule();
+		cond_resched();
 
 	} while (txcnt);