ath6kl: support changing dtim period on AP mode

This patch adds support for dtim_period configuration in beacon.

kvalo: add a comment about ignoring the error, use vif_idx,
add \n to the warning message

Signed-off-by: Etay Luz <eluz@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>