Revert "mmc: fix a race between card-detect rescan and clock-gate work instances"

This reverts commit 26fc8775b51484d8c0a671198639c6d5ae60533e, which has
been reported to cause boot/resume-time crashes for some users:

https://bbs.archlinux.org/viewtopic.php?id=118751.

Signed-off-by: Chris Ball <cjb@laptop.org>
Cc: <stable@kernel.org>