mmc: omap_hsmmc: fix cmd23 multiblock read/write

Check for set block count command fails always since host->cmd is set
to NULL in the same function incorrectly. Correct host->cmd usage properly.

Signed-off-by: Balaji T K <balajitk@ti.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Chris Ball <chris@printf.net>