wifi: ath11k: drop NULL pointer check in ath11k_update_per_peer_tx_stats()

wifi: ath11k: drop NULL pointer check in ath11k_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.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Signed-off-by: Dmitry Antipov <dmantipov@yandex.ru>
Acked-by: Jeff Johnson <quic_jjohnson@quicinc.com>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/20230906093704.14001-1-dmantipov@yandex.ru