tg3: Remove unused PHY_BRCM flags

The tg3 driver tried to communicate towards the PHY driver whether it
wanted RGMII in-band signaling enabled or disabled however there is
nothing that looks at those flags in drivers/net/phy/broadcom.c so this
does do not anything.

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