Revert "mmc: core: wait while adding MMC host to ensure root mounts successfully"

This reverts commit 3500ed90.

The reverted patch caused a significant performance regression when
booting with the root file system on eMMC.

Before the patch:

[    1.625623] VFS: Mounted root (ext4 filesystem) readonly on device 179:2.

After the patch:

[    1.935851] VFS: Mounted root (ext4 filesystem) readonly on device 179:2.

That was an addition of 310 ms which is a 19% performance degradation.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Chris Ball <cjb@laptop.org>