mmc: relocate code comment

The comment about init op being NULL used to be next to the NULL check
code. Commit 8ca51e51 ("dm: mmc: Add a way to use driver model for
MMC operations") separated the comment from the code. Put them back
together.

Fixes: 8ca51e51 ("dm: mmc: Add a way to use driver model for MMC operations")
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Reviewed-by: Peng Fan <peng.fan@nxp.com>