mmc: core: remove the MMC_DATA_STREAM flag

It's not set to MMC_DATA_STREAM anywhere.
It seems that it had been used with CMD11/CMD20.
But according to Spec, CMD11/CMD20 are obsolete command.

Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>