smsc9420: fix another postfixed timeout

Roel Kluin recently fixed several instances where variables reach -1,
but 0 is tested afterwards.  This patch fixes another, so the timeout
will be correctly detected and a warning printed.

Signed-off-by: Steve Glendinning <steve.glendinning@smsc.com>
Signed-off-by: David S. Miller <davem@davemloft.net>