mmc: omap_hsmmc: save clock rate to use in interrupt context

clk_get_rate throws DEBUG_LOCKS_WARN_ON(in_interrupt()) warning
if called from interrupt context.
use cached clock rate in set_data_timeout, so that
set_data_timeout can be called from interrupt context.

Signed-off-by: Balaji T K <balajitk@ti.com>
Signed-off-by: Chris Ball <chris@printf.net>