wil6210: support up to 20 stations in AP mode

New FW added support for upto 20 clients in AP mode. Change the driver
to support this as well. FW reports it's max supported associations in
WMI_READY_EVENT. Some WMI commands/events use cidxtid field which is
limited to 16 cids. Use new cid/tid fields instead.

For Rx packets cid from rx descriptor is limited to 3 bits (0..7),
to find the real cid, compare transmitter address with the stored
stations mac address in the driver sta array.

EDMA FW still supports 8 stations. Extending the support to 20
stations will come later.

Signed-off-by: Ahmad Masri <amasri@codeaurora.org>
Signed-off-by: Maya Erez <merez@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>