wifi: ath10k: Use list_count_nodes()

ath10k_wmi_fw_stats_num_peers() and ath10k_wmi_fw_stats_num_vdevs() really
look the same as list_count_nodes(), so use the latter instead of hand
writing it.

The first ones use list_for_each_entry() and the other list_for_each(), but
they both count the number of nodes in the list.

Compile tested only.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/e6ec525c0c5057e97e33a63f8a4aa482e5c2da7f.1682541872.git.christophe.jaillet@wanadoo.fr