8250: fix autoconfig to work with serial console

The autoconfig prints messages while holding the
port's spinlock and that causes a deadlock when
using serial console.

Signed-off-by: Flavio Leitner <fbl@redhat.com>
Acked-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>