wcn36xx: consider CTRL_EOP bit when looking for valid descriptors

In reap_tx_dxes(), when we iterate over the linked descriptors, only
consider such valid that have WCN36xx_DXE_CTRL_EOP set.

This is what the prima downstream driver is doing as well.

Signed-off-by: Daniel Mack <daniel@zonque.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>