mmc: tmio: fix clock gating on platforms with a .set_pwr() method

Do not power down the card in .set_ios(), unless MMC_POWER_OFF is
requested. This fixes the SDHI functionality on ecovec.

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