ath6kl: Define a structure for connection specific aggregation information

This patch just groups connection specific aggregation information
from struct aggr_info into a new structure (struct aggr_info_conn)
so that, in softAP mode, this can be used when each connected station
is made to have it's own aggregation state.

Signed-off-by: Vasanthakumar Thiagarajan <vthiagar@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>