VT-d: correct a comment and remove an useless if() statement

VT-d: correct a comment and remove an useless if() statement

Fix two flaws in the patch (93358e8e VT-d: introduce update_irte to update
irte safely):
1. Expand a comment in update_irte() to make it clear that VT-d hardware
doesn't update IRTE and software can't update IRTE behind us since we hold
iremap_lock.
2. remove an useless if() statement

Signed-off-by: Chao Gao <chao.gao@intel.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Kevin Tian <kevin.tian@intel.com>
Release-acked-by: Julien Grall <julien.grall@arm.com>