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

commit 9098f84cced870f54d8c410dd2444cfa61467fa0 upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>