mmc: sdhci-esdhc-imx: clear f_max in boarddata

After commit 8d86e4fc

 ("mmc: sdhci-esdhc-imx: Call mmc_of_parse()"),
it's not used anymore.

Signed-off-by: Dong Aisheng <aisheng.dong@freescale.com>
Reviewed-by: Johan Derycke <johan.derycke@barco.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>