mmc: block: Fix block status codes

Commit 2a842aca ("block: introduce new block status code type") changed
the error type but not in patches merged through the mmc tree, like
commit 0493f6fe ("mmc: block: Move boot partition locking into a driver
op"). Fix one error code that is incorrect and also use BLK_STS_OK in
preference to 0.

Fixes: 17ece345 ("Merge tag 'mmc-v4.13' of git://git.kernel.org/pub/scm/linux/kernel/git/ulfh/mmc"

)
Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>