tg3: Fix 5906 link problems

Commit 6833c043 introduced the phy
auto-powerdown capability.  While the APD feature only works for 5761
and 5784 asic revisions, the (harmless portion of the) code was applied
to all 5705 and newer devices.  However, the 5906 phy departs from the
usual design.  This commit was interfering with the 5906's ability to
negotiate link against some switches.  This patch corrects the problem.

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