wifi: ath10k: simplify ath10k_peer_create()

Use convenient 'list_count_nodes()' in 'ath10k_peer_create()',
thus making the latter a bit smaller and simpler.

Signed-off-by: Dmitry Antipov <dmantipov@yandex.ru>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/20230704180617.84948-1-dmantipov@yandex.ru