spi: fsl-cpm: substitute empty_zero_page with helper ZERO_PAGE(0)

Not all zero page implementations use empty_zero_page global pointer so
let's substitute empty_zero_page occurence with helper ZERO_PAGE(0).
Signed-off-by: Giulio Benetti <giulio.benetti@benettiengineering.com>
Link: https://lore.kernel.org/r/20221018215755.33566-2-giulio.benetti@benettiengineering.comSigned-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-fsl-cpm.c

@@ -333,7 +333,7 @@ int fsl_spi_cpm_init(struct mpc8xxx_spi *mspi)
 		goto err_bds;
 	}
 
-	mspi->dma_dummy_tx = dma_map_single(dev, empty_zero_page, PAGE_SIZE,
+	mspi->dma_dummy_tx = dma_map_single(dev, ZERO_PAGE(0), PAGE_SIZE,
 					    DMA_TO_DEVICE);
 	if (dma_mapping_error(dev, mspi->dma_dummy_tx)) {
 		dev_err(dev, "unable to map dummy tx buffer\n");