wil6210: fix dma mapping error cleanup in __wil_tx_vring_tso

In case we fail to map one of the TSO SKB fragments, we need to
clear all the mapped descriptors, from swhead to swhead+descs_used-1.

Change the desc index calculation to
i = (swhead + descs_used - 1) % vring->size;
to prevent unmpping of (swhead + descs_used) descriptor that wasn't
mapped.

Signed-off-by: Maya Erez <qca_merez@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>