Revert "mmc: enable runtime PM by default"

When SDIO runtime PM was originally introduced, we immediately faced
two regressions with two different chipsets, and in response decided
not to enable it by default.

With the recent work on the 8686 we hoped we found all the gotchas,
so 08da834a did make sense (at least experimentally).

Unfortunately we now see that some setups out there still refuse to
work when SDIO runtime PM is enabled by default (see
http://www.spinics.net/lists/linux-mmc/msg11161.html

), and obviously
we can't live with these kind of regressions.

This reverts commit 08da834a.

Signed-off-by: Ohad Ben-Cohen <ohad@wizery.com>
Cc: Daniel Drake <dsd@laptop.org>
Signed-off-by: Chris Ball <cjb@laptop.org>