net: contiki: Fix timer callback condition

etimer_expired will return true only once. This means periodic timers in
the netstack won't get to run since etimer_process calls etimer_expired
for all timers in the stack.

Jira: ZEP-475

Change-Id: I28da638789b0604f9dd0a0598a7fa590e6b0a746
Signed-off-by: Jaakko Hannikainen <jaakko.hannikainen@intel.com>