Improve some debugging output.

Replace DEBUGF calls with dprintf calls.
Make ata calls consistently use 'int' return codes.
Make each error return code in ata.c use a unique negative number.
diff --git a/src/boot.c b/src/boot.c
index 3ee957b..ff41930 100644
--- a/src/boot.c
+++ b/src/boot.c
@@ -147,7 +147,7 @@
         /* CD-ROM */
         if (! CONFIG_CDROM_BOOT)
             break;
-        u16 status = cdrom_boot();
+        int status = cdrom_boot();
         if (status) {
             printf("CDROM boot failure code : %04x\n", status);
             print_boot_failure(type, 1);