mmc: mxs-mmc: fix clock rate setting

Fix clock rate setting in the mxs-mmc driver. Previously, if div2 was 0
then the value for TIMING_CLOCK_RATE would have been 255 instead of 0.
The limits for div1 (TIMING_CLOCK_DIVIDE) and div2 (TIMING_CLOCK_RATE+1)
were also not correctly defined.

Can easily be reproduced on mx23evk: default clock for high speed sdio
cards is 50 MHz. With a SSP_CLK of 28.8 MHz default), this resulted in
an actual clock rate of about 56 kHz.  Tested on mx23evk.

Signed-off-by: Koen Beel <koen.beel@barco.com>
Reviewed-by: Wolfram Sang <w.sang@pengutronix.de>
Signed-off-by: Chris Ball <cjb@laptop.org>