[BNX2]: Add link-down workaround on 5706 serdes.

In some blade systems using the 5706 serdes, the hardware sometimes
does not properly generate link down interrupts.  We add a workaround
in the driver's timer to force a link-down when some PHY registers
report loss of SYNC.

The parallel detect logic is cleaned up slightly to better integrate
the workaround.

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