mmc: mmc_spi: Use SPI_XFER_BEGIN and SPI_XFER_END flags

Most DM based SPI host controller drivers use SPI_XFER_BEGIN and
SPI_XFER_END flags to enable/disable slave chip select.

This patch extends MMC SPI driver to pass SPI_XFER_BEGIN flag when
MMC command is send at start and pass SPI_XFER_END flag using a
dummy transfer (of bitlen = 0) at the end of MMC command.

Suggested-by: Jagan Teki <jagan@amarulasolutions.com>
Signed-off-by: Anup Patel <anup.patel@wdc.com>
Tested-by: Sagar Kadam <sagar.kadam@sifive.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>
diff --git a/drivers/mmc/mmc_spi.c b/drivers/mmc/mmc_spi.c
index f3d687a..350812a 100644
--- a/drivers/mmc/mmc_spi.c
+++ b/drivers/mmc/mmc_spi.c
@@ -84,7 +84,7 @@
 	cmdo[4] = cmdarg >> 8;
 	cmdo[5] = cmdarg;
 	cmdo[6] = (crc7(0, &cmdo[1], 5) << 1) | 0x01;
-	ret = dm_spi_xfer(dev, sizeof(cmdo) * 8, cmdo, NULL, 0);
+	ret = dm_spi_xfer(dev, sizeof(cmdo) * 8, cmdo, NULL, SPI_XFER_BEGIN);
 	if (ret)
 		return ret;
 
@@ -360,6 +360,8 @@
 	}
 
 done:
+	dm_spi_xfer(dev, 0, NULL, NULL, SPI_XFER_END);
+
 	dm_spi_release_bus(dev);
 
 	return ret;