net: emac: Fix napi poll list corruption

This patch is pretty much a carbon copy of
commit 3079c652 ("caif: Fix napi poll list corruption")
with "caif" replaced by "emac".

The commit d75b1ade ("net: less interrupt masking in NAPI")
breaks emac.

It is now required that if the entire budget is consumed when poll
returns, the napi poll_list must remain empty.  However, like some
other drivers emac tries to do a last-ditch check and if there is
more work it will call napi_reschedule and then immediately process
some of this new work.  Should the entire budget be consumed while
processing such new work then we will violate the new caller
contract.

This patch fixes this by not touching any work when we reschedule
in emac.

Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>