selftests/timers: Add missing error code assignment before test

In order to work, the 'err' return value has to be updated otherwise the
test can never be true.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Acked-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>