[prefix] Use correct register for KEEP_IT_REAL physical address conversion

Signed-off-by: Daniel Pieczko <dpieczko@solarflare.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/i386/prefix/libprefix.S

 
 	/* Convert %ds:esi and %es:edi back to physical addresses */
 	xorl	%eax, %eax
-	movw	%ds, %cx
+	movw    %ds, %ax
 	shll	$4, %eax
 	addl	%eax, %esi
 	xorl	%eax, %eax
-	movw	%es, %cx
+	movw    %es, %ax
 	shll	$4, %eax
 	addl	%eax, %edi