net: contiki: Fix the timer expiration check

We need to check the timer expiration properly. The nano_timer_test()
will return user data to caller if the timer is expired.

The issue that is fixed by this commit started to appear after
the commit 9b245204 which combined
nano_timers and nano_timeouts.

Change-Id: I9cf10e605f1c4767caa1aad9dcf29a159af55e28
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>