mmc: block: Fix 4K native sector check

The 4K native sector check does not allow for the 'do' loop nor the
variables used after the 'cmd_abort' label.

'brq' and 'req' get reassigned in the 'do' loop, so the check must not
assume what their values are. After the 'cmd_abort' label, 'mq_rq' and
'req' are used, but 'rqc' must be NULL otherwise it can be started again.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>