nano_work: Fix never yielding from fiber

If the delayed work FIFO never goes empty (e.g. because the work
callback keeps resubmitting or there's a very active ISR feeding items
there) then the fiber would never yield, causing all sorts of problems
for the system. Adding an explicit fiber_yield() call at the end of
the while-loop solves the issue.

Change-Id: I233b9fc18fc9db9172daf8689bd22d09952089cb
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>