fsl_esdhc: Only modify the field we are changing in WML

When we set the read or write watermark in WML we should maintain the
rest of the register as is, rather than using some hard coded value.

Signed-off-by: Roy Zang <tie-fei.zang@freescale.com>
Acked-by: Stefano Babic <sbabic@denx.de>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>