mmc: block: Add missing mmc_blk_put() in power_ro_lock_show()

Enclosing mmc_blk_put() is missing in power_ro_lock_show() sysfs handler,
let's add it.

Fixes: add710ea

 ("mmc: boot partition ro lock support")
Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>