ath11k: Fix the rx_filter flag setting for peer rssi stats

Set the rx_filter in ath11k_mac_config_mon_status_default(),
only when the rx_filter value exists in ath11k_debug_rx_filter().

Without this change, rx_filter gets set to 0 and peer rssi stats
aren't updating properly from firmware.

Tested-on: IPQ8074 WLAN.HK.2.1.0.1-01230-QCAHKSWPL_SILICONZ-4

Fixes: ec48d28b ("ath11k: Fix rx_filter flags setting for per peer rx_stats")

Signed-off-by: Maharaja Kennadyrajan <mkenna@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/1605091117-11005-1-git-send-email-mkenna@codeaurora.org