net/ncsi: Silence debug messages

In normal operation we see this series of messages as the host drives
the network device:

 ftgmac100 1e660000.ethernet eth0: NCSI: LSC AEN - channel 0 state down
 ftgmac100 1e660000.ethernet eth0: NCSI: suspending channel 0
 ftgmac100 1e660000.ethernet eth0: NCSI: configuring channel 0
 ftgmac100 1e660000.ethernet eth0: NCSI: channel 0 link down after config
 ftgmac100 1e660000.ethernet eth0: NCSI interface down
 ftgmac100 1e660000.ethernet eth0: NCSI: LSC AEN - channel 0 state up
 ftgmac100 1e660000.ethernet eth0: NCSI: configuring channel 0
 ftgmac100 1e660000.ethernet eth0: NCSI interface up
 ftgmac100 1e660000.ethernet eth0: NCSI: LSC AEN - channel 0 state down
 ftgmac100 1e660000.ethernet eth0: NCSI: suspending channel 0
 ftgmac100 1e660000.ethernet eth0: NCSI: configuring channel 0
 ftgmac100 1e660000.ethernet eth0: NCSI: channel 0 link down after config
 ftgmac100 1e660000.ethernet eth0: NCSI interface down
 ftgmac100 1e660000.ethernet eth0: NCSI: LSC AEN - channel 0 state up
 ftgmac100 1e660000.ethernet eth0: NCSI: configuring channel 0
 ftgmac100 1e660000.ethernet eth0: NCSI interface up

This makes all of these messages netdev_dbg. They are still useful to
debug eg. misbehaving network device firmware, but we do not need them
filling up the kernel logs in normal operation.

Acked-by: Samuel Mendoza-Jonas <sam@mendozajonas.com>
Signed-off-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: David S. Miller <davem@davemloft.net>