ath6kl: implement ethtool stats

This supports a way to get target stats through normal
ethtool stats API.

For instance:
# ethtool -S wlan1
NIC statistics:
     tx_pkts_nic: 353
     tx_bytes_nic: 25142
     rx_pkts_nic: 6
     rx_bytes_nic: 996
     d_tx_ucast_pkts: 89
     d_tx_bcast_pkts: 264
     d_tx_ucast_bytes: 3020
     d_tx_bcast_bytes: 22122
...

Signed-off-by: Ben Greear <greearb@candelatech.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>