dwmmc: Properly implement power_off/power_up

Write to the PWREN register should be done in update_ios based
on the power_mode value in the ios struct.
Also none of the manual (RockChip and Altera) and Linux talks about
the needed for an inverted PWREN value so just remove this.
This fixes eMMC (and possibly SD) when u-boot didn't setup the controller.

Reported by: avg
Tested-on: Rock64, RockPro64