ALSA: line6: add hw monitor volume control for POD HD500

Add hw monitor volume control for POD HD500. The same change may
work for HD500X but I don't have it to test.

Signed-off-by: Vasily Khoruzhick <anarsoul@gmail.com>
Link: https://lore.kernel.org/r/20200713152852.65832-1-anarsoul@gmail.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>
diff --git a/sound/usb/line6/driver.h b/sound/usb/line6/driver.h
index ec20fbe..71d3da1 100644
--- a/sound/usb/line6/driver.h
+++ b/sound/usb/line6/driver.h
@@ -108,6 +108,8 @@ enum {
 	LINE6_CAP_CONTROL_MIDI = 1 << 4,
 	/* device provides low-level information */
 	LINE6_CAP_CONTROL_INFO = 1 << 5,
+	/* device provides hardware monitoring volume control */
+	LINE6_CAP_HWMON_CTL =	1 << 6,
 };
 
 /*
@@ -185,6 +187,8 @@ extern int line6_read_data(struct usb_line6 *line6, unsigned address,
 			   void *data, unsigned datalen);
 extern int line6_read_serial_number(struct usb_line6 *line6,
 				    u32 *serial_number);
+extern int line6_send_raw_message(struct usb_line6 *line6,
+					const char *buffer, int size);
 extern int line6_send_raw_message_async(struct usb_line6 *line6,
 					const char *buffer, int size);
 extern int line6_send_sysex_message(struct usb_line6 *line6,