ath6kl: Fix key configuration to copy at most seq_len from seq

There is no guarantee on the caller using 8-octet buffer for
key->seq, so better follow the key->seq_len parameter on figuring
out how many octets to copy.

Signed-off-by: Jouni Malinen <jouni@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>