xen/events: drop negativity check of unsigned parameter

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=80531


Reported-by: David Binderman <dcb314@hotmail.com>
Signed-off-by: Andrey Utkin <andrey.krieger.utkin@gmail.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>