dsa: sja1105: dynamically allocate stats structure

The addition of sja1105_port_status_ether structure into the
statistics causes the frame size to go over the warning limit:

drivers/net/dsa/sja1105/sja1105_ethtool.c:421:6: error: stack frame size of 1104 bytes in function 'sja1105_get_ethtool_stats' [-Werror,-Wframe-larger-than=]

Use dynamic allocation to avoid this.

Fixes: 336aa67b

 ("net: dsa: sja1105: show more ethtool statistics counters for P/Q/R/S")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>