sd: implement sd_check_events()

Replace sd_media_change() with sd_check_events().  sd used to set the
changed state whenever the device is not ready, which can cause event
loop while the device is not ready.  Media presence handling code is
changed such that the changed state is set iff the media presence
actually changes.  UA still always sets the changed state and
NOT_READY always (at least where it used to set ->changed) clears
media presence, so no event is lost.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Kay Sievers <kay.sievers@vrfy.org>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>