forcedeth: Stay in NAPI as long as there's work

The following does the same thing without the extra overhead
of testing all the registers. It also handles the out of memory
case.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Tested-by: Tom Herbert <therbert@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>