tg3: 5715 does not link up when autoneg off

Commit d13ba512

 ("tg3: Remove
SPEED_UNKNOWN checks") cleaned up the autoneg advertisement by
removing some dead code. One effect of this change was that the
advertisement register would not be updated if autoneg is turned off.

This exposed a bug on the 5715 device w.r.t linking. The 5715 defaults
to advertise only 10Mb Full duplex. But with autoneg disabled, it needs
the configured speed enabled in the advertisement register to link up.

This patch adds the work around to advertise all speeds on the 5715 when
autoneg is disabled.

Reported-by: Marcin Miotk <marcinmiotk81@gmail.com>
Reviewed-by: Benjamin Li <benli@broadcom.com>
Signed-off-by: Nithin Nayak Sujir <nsujir@broadcom.com>
Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>