net: phy: bcm7xxx: do not use PHY_BRCM_100MBPS_WAR

There is no need for the PHY driver to check PHY_BRCM_100MBPS_WAR since
that is redundant with checking the PHY device supported features. Get
rid of that workaround flag.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>