dm: mmc_spi: Fix NULL pointer dereference in mmc_spi_bind()

The mmc_spi driver's priv is not available in its bind phase(). Use
platdata instead.

Fixes: 05e35d42 ("mmc: mmc_spi: Re-write driver using DM framework")
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>