wil6210: add scatter-gather support

When setting fragmented skb for Tx, assign skb to the last descriptor
and set number of fragments in the 1-st one
On Tx complete, HW sets "DU" bit in Tx descriptor only for the last
descriptor; so search for it using number of fragments field.
Middle descriptors may have "DU" bit not set by the hardware.

Signed-off-by: Vladimir Kondratiev <qca_vkondrat@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/ath/wil6210/wil6210.h

@@ -214,6 +214,7 @@ struct pending_wmi_event {
  */
 struct wil_ctx {
 	struct sk_buff *skb;
+	u8 nr_frags;
 	u8 mapped_as_page:1;
 };