[romprefix] Do not preserve unused register %di

Since some PnP BIOSes fail to set %es:di to point to the PnP signature
on entry, we identify a PnP BIOS by scanning through the top 64kB of
base memory looking for the PnP structure.  We therefore don't
actually use the values of %es:di provided to the initialisation entry
point, and so there is no need to preserve them.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/i386/prefix/romprefix.S

 	pushw	%cs
 	popw	%ds
 
-	/* Shuffle some registers around.  We need %di available for
-	 * the print_xxx functions, and in a register that's
-	 * addressable from %es, so shuffle as follows:
-	 *
-	 *    %di (pointer to PnP structure) => %bx
-	 *    %bx (runtime segment address, for PCI 3.0) => %gs
-	 */
+	/* Store PCI 3.0 runtime segment address for later use */
 	movw	%bx, %gs
-	movw	%di, %bx
 
 	/* Store PCI bus:dev.fn address */
 	movw	%ax, init_pci_busdevfn