uwb: fix races between events and neh timers

Always use del_timer_sync() before freeing nehs.  Destroy all nehs after
stopping the radio controller and before cleaning up the reservation
manager.  Handle the timer running after an event has removed the neh.

This fixes various oopses that may occur if a radio controller is removed
while a neh timer is still active.

Signed-off-by: David Vrabel <david.vrabel@csr.com>