rtw88: pci: reset ring index when release skbs in tx ring

When skbs queued for each TX ring are relased in PCI stop flow,
the ring index should be reset at the same time.

Signed-off-by: Chin-Yen Lee <timlee@realtek.com>
Signed-off-by: Yan-Hsuan Chuang <yhchuang@realtek.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>