Xen: fix typo in previous patch

Xen: fix typo in previous patch

Correctly name the irq_chip structure to fix an immediate failure when booting
as a xen pv_ops guest with a NULL pointer exception. The missing 'x' was
introduced in commit [fb412a178502dc498430723b082a932f797e4763] applied to
2.6.3[25]-stable trees.  The commit to mainline was
[aaca49642b92c8a57d3ca5029a5a94019c7af69f] which did not have the problem.

Signed-off-by: James Dingwall <james@dingwall.me.uk>
Reported-by: Pawel Zuzelski <pawelz@pld-linux.org>
Tested-by: Pawel Zuzelski <pawelz@pld-linux.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>