mmc: wmt-sdmmc: Simplify wmt_set_sd_power implementation

Simplify the code to set/clear BM_SD_OFF bit:

          | power_inverted: 0  | power_inverted: 1
-------------------------------------------------
enable: 0 | SET BM_SD_OFF      | CLEAR BM_SD_OFF
-------------------------------------------------
enable: 1 | CLEAR BM_SD_OFF    | SET BM_SD_OFF

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Tony Prisk <linux@prisktech.co.nz>
Signed-off-by: Chris Ball <cjb@laptop.org>