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>

include/fsl_esdhc.h

@@ -133,6 +133,8 @@
 
 #define WML		0x2e044
 #define WML_WRITE	0x00010000
+#define WML_RD_WML_MASK	0xff
+#define WML_WR_WML_MASK	0xff0000
 
 #define BLKATTR		0x2e004
 #define BLKATTR_CNT(x)	((x & 0xffff) << 16)