sh: ecovec: switch SDHI controllers to card polling

Polling for card hotplug events is a better option than using SDHI native
hotplug interrupt, because the latter option forces the controller to stay
permanently powered up and enabled.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>