tg3: Add support for link flap avoidance

tg3: Add support for link flap avoidance

This patch and the following two patches add support for link flap avoidance
by maintaining the link on power down. This feature is required for
management capable devices to have the management connection
uninterrupted on driver reload, reboot and interface up/down.

The other pros of this feature are
 - It speeds up boot up time by several seconds as DHCP addresses can be
   acquired faster.
 - It avoids lengthy Spanning Tree delay.

On powerup the hardware brings up the phy with default settings. If the
link is not up, the management software configures the phy to gigabit
and starts autonegotiate. Subsequently, as long as the link is up, the
driver and management refrain from resetting and/or changing any
configuration that the link depends on.

The LNK_FLAP_AVOID setting is an NVRAM user configurable bit and is
disabled by default.  If this setting is enabled, we skip powering down
the phy and resetting it.

A second NVRAM setting is 1G_ON_VAUX_OK (off by default). This adds
support for gigabit link speed when device is on auxiliary power.

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>