um: fix os_timer_one_shot()

os_timer_one_shot() gets passed a value "unsigned long delta",
so must not have an "int ticks" as that actually ends up being
-1, and thus triggering a timer over and over again.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Richard Weinberger <richard@nod.at>