mmc: sdhci-esdhc-imx: add SDHCI_TRANSFER_MODE read function

Used to read out the correct value of SDHCI_TRANSFER_MODE register
for upper layer.

Signed-off-by: Dong Aisheng <b29396@freescale.com>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Chris Ball <cjb@laptop.org>