net: dsa: qca8k: Force CPU port to its highest bandwidth

By default autonegotiation is enabled to configure MAC on all ports.
For the CPU port autonegotiation can not be used so we need to set
some sensible defaults manually.

This patch forces the default setting of the CPU port to 1000Mbps/full
duplex which is the chip maximum capability.

Also correct size of the bit field used to configure link speed.

Fixes: 6b93fb46

 ("net-next: dsa: add new driver for qca8xxx family")
Signed-off-by: Michal Vokáč <michal.vokac@ysoft.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>