wifi: ath12k: drop NULL pointer check in ath12k_update_per_peer_tx_stats()

Since 'user_stats' is a fixed-size array of 'struct htt_ppdu_user_stats'
in 'struct htt_ppdu_stats', any of its member can't be NULL and so
relevant check may be dropped.

Signed-off-by: Ma Ke <make_ruc2021@163.com>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/20231013074711.2202850-1-make_ruc2021@163.com