forcedeth: fix race when unloading module

When forcedeth module is unloaded, there exists a path that can lead
to mod_timer() after del_timer_sync(), causing an oops. This patch
short-circuits this unneeded path, which originates in
nv_get_ethtool_stats().

Tested:
  x86_64 16-way + 3 ethtool -S infinite loops + 100Mbps incoming traffic
  + rmmod/modprobe/ifconfig in a loop

Initial-Author: Salman Qazi <sqazi@google.com>
Discussion: http://patchwork.ozlabs.org/patch/123548/

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