mmc: sdhci-esdhc-imx: fix incorrect max timeout cout for uSDHC

The default sdhci code use the 1 << 27 as the max timeout counter to
to calculate the max_busy_timeout, however it's not correct for uSDHC
since its the max counter is 1 << 28.
Implement esdhc_get_max_timeout_cout to handle it correctly.

Reviewed-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Dong Aisheng <b29396@freescale.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>