ath6kl: Make sure to delete rx aggregation timer in aggr_reset_state()

The timer which is used to flush rx aggregation frames needs to
be disabled when resetting the aggregation state. This is found
in code review.

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