phy: Fix warning in get_phy_device().

drivers/net/phy/phy_device.c: In function ‘get_phy_device’:
drivers/net/phy/phy_device.c:340:14: warning: ‘phy_id’ may be used uninitialized in this function [-Wmaybe-uninitialized]

GCC can't see that when we return zero we always initialize
phy_id and that's the only path where we use it.

Initialize phy_id to zero to shut it up.

Signed-off-by: David S. Miller <davem@davemloft.net>