mmc: sdhci-esdhc-imx: set back the burst_length_enable bit to 1
Currently we find that if a usdhc is choosed to boot system, then ROM
code will set the burst length enable bit of this usdhc as 0.
This will make performance drop a lot if this usdhc's burst length is
configed. So this patch set back the burst_length_enable bit as 1,
which is the default value, and means burst length is enabled for INCR.