Bluetooth: btusb: fix bt fiwmare downloading failure issue for qca btsoc.

stable inclusion
from linux-4.19.198
commit 012a3944008fc774b1eae42be7e8dfc4ee2ab521

--------------------------------

[ Upstream commit 4f00bfb3 ]

This is btsoc timing issue, after host start to downloading bt firmware,
ep2 need time to switch from function acl to function dfu, so host add
20ms delay as workaround.
Signed-off-by: NTim Jiang <tjiang@codeaurora.org>
Signed-off-by: NMarcel Holtmann <marcel@holtmann.org>
Signed-off-by: NSasha Levin <sashal@kernel.org>
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>

drivers/bluetooth/btusb.c

@@ -2595,6 +2595,11 @@ static int btusb_setup_qca_download_fw(struct hci_dev *hdev,
 	sent += size;
 	count -= size;
 
+	/* ep2 need time to switch from function acl to function dfu,
+	 * so we add 20ms delay here.
+	 */
+	msleep(20);
+
 	while (count) {
 		size = min_t(size_t, count, QCA_DFU_PACKET_LEN);