rsi: add firmware support for AP+BT dual mode

Currently, AP mode will work on only WLAN alone firmware. To give support
for AP and BT dual mode, adding firmware entry in 'struct ta_metadata'.
The firmware entry is based on what coex_mode is used in driver and coex
mode '4' for all AP+BT related functionalities. Hence, added the same.

Signed-off-by: Siva Rebbagondla <siva.rebbagondla@redpinesignals.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>