Move the ksCurTime assertions out of setDeadline, because they are not
necessarily true there. Assert ksCurTime in setNextInterrupt instead.

We only know that the deadline being set is at least ksCurTime -
getTimerPrecision(), which can be slightly in the past (ksCurTime is
already slightly in the past, at kernel entry).

Signed-off-by: Gerwin Klein <gerwin.klein@proofcraft.systems>