mmc: sdhci-esdhc-imx: correct the max timeout count

i.MX USDHC Reference Manual has a mistake, for the register SYS_CTRL,
the DTOCV(bit 19~16) means the data timeout counter value. When DTOCV
is set to 0xF, it means SDCLK << 29, not SDCLK << 28.

Signed-off-by: Haibo Chen <haibo.chen@nxp.com>
Acked-by: Dong Aisheng <aisheng.dong@nxp.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>