ufddos80.asm: add XD to the DOS device table

Add XD (nonstandard second hard disk) to the DOS device table; there
are two empty slots. This does NOT add the BASIC device nodes for
them, since that would take memory (8*4 = 32 bytes.)

Signed-off-by: H. Peter Anvin <hpa@zytor.com>

rv32/roms/abc80/asmsrc/ufddos80.asm

@@ -1589,9 +1589,9 @@ devdes_rom:     ;; 678a <- 6069 6560
                 defb 0x40 + 46          ; 679a n    6e
                 defb 2                  ; 679b .    02
                 .ascii "SF"             ; 679c SF   53 46
-                defb 0x00               ; 679e .    00
-                defb 0                  ; 679f .    00
-                .ascii "??"             ; 67a0 ??   3f 3f
+                defb 37                 ; Nonstandard second hard drive XD_:
+                defb 5
+                .ascii "XD"
                 defb 0x00               ; 67a2 .    00
                 defb 0                  ; 67a3 .    00
                 .ascii "??"             ; 67a4 ??   3f 3f