mmc: mmc_ops: fix a typo for comment of mmc_start_bkops

Just a trivial fix for that found by reading the code.

Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>