[romprefix] Fix ROM image copy on PCI 3.0 BIOSes

Add the missing instructions to the code path that I never tested due
to not having a PCI 3.0 system available.

(D'oh)

src/arch/i386/prefix/romprefix.S

 	movzbw	romheader_size, %cx
 	shlw	$9, %cx
 	movw	%ax, %es
-	rep	movsb
+	xorw	%si, %si
+	xorw	%di, %di
+	cs rep	movsb
 no_pci3:
 no_pci:
 	/* Prompt for POST-time shell */