mmc: sh_mobile_sdhi: simplify code for voltage switching

A last minute fix applied by Ulf made room for some simplification.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>