net: phylink: make Broadcom BCM84881 based SFPs work

The Broadcom BCM84881 does not appear to send the SGMII control word
when operating in SGMII mode, which causes network adapters to fail
to link with the PHY, or decide to operate at fixed 1G speed, even if
the PHY negotiated 100M.

Work around this by detecting the Broadcom BCM84881 and switch to phy
mode rather than inband mode.

Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>