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.