ath6kl: Export beacon interval and DTIM period through STA info

Now that we allow the first Beacon frame after each connection to
be processed at the host, we can figure out the DTIM period and
expose it with Beacon interval through STA info BSS parameters
to user space.

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