mmc: sdhci-esdhc-imx: fix timeout on i.MX's sdhci

This patch fixes timeout problems on i.MX's sdhci as suggested by
Richard Zhu.

Tested on:
- i.MX257: not needed
- i.MX357: needed
- i.MX515: needed

More details can be found here:
http://lists.infradead.org/pipermail/linux-arm-kernel/2010-October/029748.html

Signed-off-by: Eric Bénard <eric@eukrea.com>
Tested-by: Shawn Guo <shawn.gsc@gmail.com>
Acked-by: Wolfram Sang <w.sang@pengutronix.de>
Signed-off-by: Chris Ball <cjb@laptop.org>