net: phy: bcm54140: fix phy_id_mask

Broadcom defines the bits for this PHY as follows:
  { oui[24:3], model[6:0], revision[2:0] }

Thus we have to mask the lower three bits only.

Fixes: 6937602e ("net: phy: add Broadcom BCM54140 support")
Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>