tg3: Fix failure to enable WoL by default when possible

tg3 is supposed to enable WoL by default on adapters which support
that, but it fails to do so unless the adapter's
/sys/devices/.../power/wakeup file contains 'enabled' during the
initialization of the adapter.  Fix that by making tg3 use
device_set_wakeup_enable() to enable wakeup automatically whenever
WoL should be enabled by default.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Signed-off-by: David S. Miller <davem@davemloft.net>