Fix the rx refill timeout to only refill rings that are currently empty.

Fix the rx refill timeout to only refill rings that are currently empty.
This is normally how the timeout works, but in this driver it's complicated
by the use of separate rx and ag rings.

This prevents the refill timeout from operating on a ring concurrently
with an rx interrupt, which leads to corruption and crashes as experienced
by bluhm@

ok dlg@
earlier version tested by and ok bluhm@