ar71xx: fix HiveAP 121 PLL for 1000M
The Aerohive HiveAP 121 has the wrong PLL value set for Gigabit speeds,
leading to packet-loss. 10M and 100M work fine.
This commit sets the Gigabit Ethernet PLL value to the correct value,
fixing packet loss.
Confirmed with iperf and floodping.
Signed-off-by: David Bauer <[email protected]>