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>