mmc: debugfs: correct wrong voltage value

Correct the wrong voltage value shown in debugfs for mmc/sd/sdio.

Signed-off-by: Chuanxiao Dong <chuanxiao.dong@intel.com>
Signed-off-by: Pawel Wodkowski <pawelx.wodkowski@intel.com>
Fixes: 42cd95a0 ("mmc: core: debugfs: Add signal_voltage to ios dump")
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>