mmc: block: Fix tag condition with packed writes

Apparently a cut-and-paste error, 'do_data_tag' is using 'brq' for data
size even though 'brq' has not been set up. Instead use blk_rq_sectors().

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Reviewed-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>