mmc: omap_hsmmc: cleanup clock configuration

Add a separate function for starting the clock, stopping the clock and
setting the clock. Starting the clock and stopping the clock can
be used irrespective of setting the clock (For example during iodelay
recalibration).
Also set the clock only if there is a change in frequency.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>