mmc: omap: remove clock rate hard coding

MMC master clock rate can vary for each instance of the MMC controller
on the device. Use clk_get_rate instead to get the value.

Signed-off-by: Balaji TK <balajitk@ti.com>
Reviewed-by: Venkatraman S <svenkatr@ti.com>
Tested-by: Hebbar, Gururaja <gururaja.hebbar@ti.com>
Acked-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Signed-off-by: Chris Ball <cjb@laptop.org>