rsi: fix for wowlan wakeup failure

During wowlan regression tests, sometimes radio is not waking up for wowlan
packet in coex mode. This is because of power save is enabled by default
in case of coex mode greater than one. Hence, disable power save in coex
mode to avoid radio loss.

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