don't let BCM63XX_PHY depend on non-existant symbol

The kernel doesn't have a symbol called BCM63XX.  There is a symbol
BCM63XX_ENET (introduced in 9b1fc55a, 6 weeks after 09bb9aa0 that
introduced BCM63XX_PHY), but the driver compiles without that, too.

Cc: Maxime Bizon <mbizon@freebox.fr>
Cc: Florian Fainelli <florian@openwrt.org>
Cc: David S. Miller <davem@davemloft.net>
Cc: Ralf Baechle <ralf@linux-mips.org>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>