mmc: sdhci: implement the .card_event() method

Extracting a part of the SDHCI card tasklet into a .card_event()
implementation allows SDHCI hosts to use generic card-detection
services, e.g. the GPIO slot function.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Reviewed-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Chris Ball <cjb@laptop.org>