kgdb/x86: Fix reporting of 'si' in kgdb on x86_64

This patch fixes an error in kgdb for x86_64 which would report
the value of dx when asked to give the value of si.

Signed-off-by: Steffen Liebergeld <steffen.liebergeld@kernkonzept.com>
Cc: Jason Wessel <jason.wessel@windriver.com>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>