kernel: timeout: Fix adding of an absolute timeout

Correct the way the relative ticks value is calculated for an absolute
timeout. Previously, elapsed() was called twice and the returned value
was first subtracted from and then added to the ticks value. It could
happen that the HW counter value read by elapsed() changed between the
two calls to this function. This caused the test_timeout_abs test case
from the timer_api test suite to occasionally fail, e.g. on certain nRF
platforms.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>