tg3: Refine power management and WOL code

Commit 12dac075

 ("tg3: adapt tg3 to
use reworked PCI PM code") introduced the new PCI PM API to the tg3
driver.  The patch was understandably conservative, so this patch
elaborates on that work.

The patch starts by creating a single point in tg3_set_power_state()
to decide whether or not to enable WOL.  The rest of the code in
tg3_set_power_state() was then pivoted to use the result of this
decision.

The patch then makes sure the device is allowed to wakeup before
reporting whether or not WOL is currently enabled.  The final hunks of
the patch consolidate where the WOL capability and WOL enabled flags
are set to a single location.

Signed-off-by: Matt Carlson <mcarlson@broadcom.com>
Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>