mmc: sdhci-esdhc-imx: remove "WP" from flag ESDHC_FLAG_GPIO_FOR_CD_WP

The use of flag ESDHC_FLAG_GPIO_FOR_CD_WP is all CD related.  It does
not necessarily need to bother WP in the flag name.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Chris Ball <cjb@laptop.org>