tg3: Minor EEE code tweaks

The first hunk of this patch makes sure that the driver checks for the
appropriate preconditions before checking if EEE negotiation succeeded.
More specifically the link needs to be full duplex for EEE to be
enabled.

The second and third hunks of this patch fix a bug where the eee
advertisement register would be programmed with extra bits set.

The fourth hunk of this patch makes sure the EEE capability flag is not
set for 5718 A0 devices and that the device is not a serdes device.

None of these modifications are strictly necessary.  The driver /
hardware still does the right thing.  They are submitted primarily for
correctness.

Signed-off-by: Matt Carlson <mcarlson@broadcom.com>
Reviewed-by: Benjamin Li <benli@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>