forcedeth: stats updated with a deferrable timer

Mark stats timer as deferrable: punctuality in waking the stats timer
callback doesn't matter much, as it is responsible only to avoid
integer wraparound.

We need at least 1 other timer to fire within 17s (fully loaded 1Gbps)
to avoid wrap-arounds. Desired period is still 10s.

Signed-off-by: David Decotigny <david.decotigny@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>