mmc: remove the hard setting for tran_speed

mmc_set_clock is set to the hard-coding.
But i think good that use the tran_speed value.

Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Andy Fleming <afleming@freescale.com>