wifi: wilc1000: set station_info flag only when signal value is valid

Set station_info->filled to indicate signal level only when its value is
received successfully using wilc_get_rssi().

Signed-off-by: Ajay Singh <ajay.kathat@microchip.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20220720160302.231516-4-ajay.kathat@microchip.com