drivers: nrf_wifi: Fix the issue of delayed keepalive

Supplicant sends keepalive on timer expiry but due to driver + nRF70
delays it is delayed to the AP, meanwhile some APs disconnect due to
lack of keepalive.

Set the processing delay to 700ms (based on tests) to fix.

Signed-off-by: Chaitanya Tata <Chaitanya.Tata@nordicsemi.no>