wil6210: optimize index manipulation in wil_vring_reap_rx

Use temporal variable for often used vring->swhead;
and use proper index in debug printing - vring->swhead
used before was modified in wil_vring_advance_head
and then increased value was used in debug print

Signed-off-by: Vladimir Kondratiev <qca_vkondrat@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>