IDE: fix compiler warnings

The changes introduced by commit 0abddf82 ``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>