Added some things that were missing from the DMA controller;

Fixed a link-breaking bug in cdrom.c;
The AHA-154x now supports commands 0x28 (GET EXTENDED BIOS INFO) and 0x29 (UNLOCK MAILBOX) and no longer reports a firmware revision with scatter/gather bug;
The BusLogic now supports command 0x20 (RESET);
Fixed the GPIO register mapping on the National Semiconductors PC87306 Super I/O chip.

src/ibm.h

@@ -308,6 +308,7 @@ typedef struct DMA
         uint8_t page[4];
         uint8_t stat;
         uint8_t command;
+        uint8_t request;
 } DMA;
 
 DMA dma,dma16;