IDE: fix compiler warnings

The changes introduced by commit 0abddf8 ``cmd_ide: enhance new
feature "CONFIG_IDE_AHB"'' caused compiler warnings like

cmd_ide.c: In function 'ide_init':
cmd_ide.c:716: warning: assignment from incompatible pointer type

Constify the respective function arguments to fix this.

Signed-off-by: Wolfgang Denk <wd@denx.de>
diff --git a/include/ide.h b/include/ide.h
index 80a10f4..8ecc9dd 100644
--- a/include/ide.h
+++ b/include/ide.h
@@ -52,7 +52,7 @@
 
 void ide_init(void);
 ulong ide_read(int device, lbaint_t blknr, ulong blkcnt, void *buffer);
-ulong ide_write(int device, lbaint_t blknr, ulong blkcnt, void *buffer);
+ulong ide_write(int device, lbaint_t blknr, ulong blkcnt, const void *buffer);
 
 #if defined(CONFIG_OF_IDE_FIXUP)
 int ide_device_present(int dev);