USB: usblp: fix DMA to stack

Stack-allocated buffers cannot be used for DMA (on all architectures).

Replace the HP-channel macro with a helper function that allocates a
dedicated transfer buffer so that it can continue to be used with
arguments from the stack.

Note that the buffer is cleared on allocation as usblp_ctrl_msg()
returns success also on short transfers (the buffer is only used for
debugging).

Cc: [e-mail]
Signed-off-by: Johan Hovold <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/usb/class/usblp.c

···
 #define usblp_reset(usblp)\
 	usblp_ctrl_msg(usblp, USBLP_REQ_RESET, USB_TYPE_CLASS, USB_DIR_OUT, USB_RECIP_OTHER, 0, NULL, 0)
 
-#define usblp_hp_channel_change_request(usblp, channel, buffer) \
-	usblp_ctrl_msg(usblp, USBLP_REQ_HP_CHANNEL_CHANGE_REQUEST, USB_TYPE_VENDOR, USB_DIR_IN, USB_RECIP_INTERFACE, channel, buffer, 1)
+static int usblp_hp_channel_change_request(struct usblp *usblp, int channel, u8 *new_channel)
+{
+	u8 *buf;
+	int ret;
+
+	buf = kzalloc(1, GFP_KERNEL);
+	if (!buf)
+		return -ENOMEM;
+
+	ret = usblp_ctrl_msg(usblp, USBLP_REQ_HP_CHANNEL_CHANGE_REQUEST,
+			USB_TYPE_VENDOR, USB_DIR_IN, USB_RECIP_INTERFACE,
+			channel, buf, 1);
+	if (ret == 0)
+		*new_channel = buf[0];
+
+	kfree(buf);
+
+	return ret;
+}
 
 /*
  * See the description for usblp_select_alts() below for the usage