ath10k: wmi: Convert use of 6 to ETH_ALEN

Use the appropriate define instead of 6.

Signed-off-by: Joe Perches <joe@perches.com>
Noticed-by: Julia Lawall <julia.lawall@lip6.fr> via spatch script
Signed-off-by: David S. Miller <davem@davemloft.net>