mmc: use sysfs groups to handle conditional attributes

Suppressing uevents turned out to be a bad idea as it screws up the
order of events, making user space very confused. Change the system to
use sysfs groups instead.

This is a regression that, for some odd reason, has gone unnoticed for
some time. It confuses hal so that the block devices (which have the
mmc device as a parent) are not registered. End result being that
desktop magic when cards are inserted won't work.

Signed-off-by: Pierre Ossman <drzeus@drzeus.cx>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>