net: bcmgenet: remove PHY_BRCM_100MBPS_WAR

Now that we have removed the need for the PHY_BRCM_100MBPS_WAR flag, we
can remove it from the GENET driver and the broadcom shared header file.
The PHY driver checks the PHY supported bitmask instead.

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