mmc: sdhi: do not install dummy callbacks

Currently the SDHI glue for the TMIO MMC driver installs dummy .get_cd() and
.set_pwr() callbacks even if the platform didn't supply them. This is not
necessary, since the TMIO MMC driver itself checks for NULL callbacks. This
is also dubious if the platform provides a regulator for SD-card power
switching. It is better to only install those callbacks, if they are really
provided by the platform.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Chris Ball <cjb@laptop.org>