tg3: Report invalid link from tg3_get_settings()

Currently the tg3 driver leaves the speed and duplex fields
uninitialized in tg3_get_settings() if the device is not up.  This can
lead to some strange deductions in certain versions of ethtool.  When
the device is up and the link is down, the driver reports SPEED_INVALID
and DUPLEX_INVALID for these fields.  This patch makes the presentation
consistent by returning SPEED_INVALID and DUPLEX_INVALID when the device
has not been brought up as well.

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