mmc: sh_mmcif: remove redundant .down_pwr() callback

From the original version of sh_mmcif the .set_pwr() callback has only been
used to turn the card's power on, and the .down_pwr() callback has been
used to turn it off. .set_pwr() can be used for both these tasks, which is
also how it is implemented by the only user of this API: the SH7724 ecovec
board.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Reviewed-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Chris Ball <cjb@laptop.org>