mmc: sdhci: use f_max instead of host->clock for timeouts

When timeout_clk is calculated the host->clock could be zero.
So, instead of host->clock the calculation now uses mmc->f_max.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Chris Ball <cjb@laptop.org>