Fix clock handling accordingly to PXA manual :
- enable the MMC controller clock once and for all
- only disable the MMC bus clock when changing the MMCLK by
adjusting the clock ratio, else let the controller and SD
card shut down the clock as the see fit.
Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>