mmc: block: Add error handling support for add_disk()

We never checked for errors on add_disk() as this function returned void.
Now that this is fixed, use the shiny new error handling.

The caller only cleanups the disk if we pass on an allocated md but on
error we return return ERR_PTR(ret), and so we must do all the unwinding
ourselves.

Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
Link: https://lore.kernel.org/r/20210830212538.148729-5-mcgrof@kernel.org
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>