um: irqs: allow invoking time-travel handler multiple times

If we happen to get multiple messages while IRQS are already
suspended, we still need to handle them, since otherwise the
simulation blocks.

Remove the "prevent nesting" part, time_travel_add_irq_event()
will deal with being called multiple times just fine.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Richard Weinberger <richard@nod.at>