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>

drivers/mmc/card/block.c

@@ -208,6 +208,8 @@ static ssize_t power_ro_lock_show(struct device *dev,
 
 	ret = snprintf(buf, PAGE_SIZE, "%d\n", locked);
 
+	mmc_blk_put(md);
+
 	return ret;
 }