mmc: block: Init ro_lock sysfs attr to fix lockdep warnings

Signed-off-by: Rabin Vincent <rabin.vincent@stericsson.com>
Signed-off-by: Johan Rudholm <johan.rudholm@stericsson.com>
Signed-off-by: Ulf Hansson <ulf.hansson@stericsson.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Namjae Jeon <linkinjeon@gmail.com>
Signed-off-by: Chris Ball <cjb@laptop.org>