wifi: ath12k: Fix memory leak due to multiple rx_stats allocation

wifi: ath12k: Fix memory leak due to multiple rx_stats allocation

rx_stats for each arsta is allocated when adding a station.
arsta->rx_stats will be freed when a station is removed.

Redundant allocations are occurring when the same station is added
multiple times. This causes ath12k_mac_station_add() to be called
multiple times, and rx_stats is allocated each time. As a result there
is memory leaks.

Prevent multiple allocations of rx_stats when ath12k_mac_station_add()
is called repeatedly by checking if rx_stats is already allocated
before allocating again. Allocate arsta->rx_stats if arsta->rx_stats
is NULL respectively.

Tested-on: QCN9274 hw2.0 PCI WLAN.WBE.1.3.1-00173-QCAHKSWPL_SILICONZ-1
Tested-on: WCN7850 hw2.0 PCI WLAN.HMT.1.0.c5-00481-QCAHMTSWPL_V1.0_V2.0_SILICONZ-3

Signed-off-by: Sidhanta Sahu <sidhanta.sahu@oss.qualcomm.com>
Signed-off-by: Muna Sinada <muna.sinada@oss.qualcomm.com>
Reviewed-by: Mahendran P <quic_mahep@quicinc.com>
Link: https://patch.msgid.link/20250326213538.2214194-1-muna.sinada@oss.qualcomm.com
Signed-off-by: Jeff Johnson <jeff.johnson@oss.qualcomm.com>