tg3: Remove SPEED_UNKNOWN checks

tg3_phy_copper_begin() has code that configures the link
advertisements through the use of the link_config.speed and
link_config.duplex members.  The driver does not internally use these
members in this way, nor is it (currently) permitted via the ethtool
interface.  This patch removes the dead code.

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