ALSA: usb-audio: Fix Focusrite Scarlett 6i6 gen1 - input handling

The Scarlett 6i6 has no padding on rear inputs 3/4 but a gainstage.
This patch introduces this functionality as to be seen in the mac
or windows scarlett control.

The correct address could already be found in the dump info, but was
never used. Without this patch inputs 3/4 are quite unusable else.

Signed-off-by: Jens Verwiebe <info@jensverwiebe.de>
Link: https://lore.kernel.org/r/384d65cd-5e87-91eb-9fc3-e57226f534c6@jensverwiebe.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/usb/mixer_scarlett.c

@@ -142,6 +142,7 @@ enum {
 	SCARLETT_OUTPUTS,
 	SCARLETT_SWITCH_IMPEDANCE,
 	SCARLETT_SWITCH_PAD,
+	SCARLETT_SWITCH_GAIN,
 };
 
 enum {
@@ -192,6 +193,15 @@ static const struct scarlett_mixer_elem_enum_info opt_pad = {
 	}
 };
 
+static const struct scarlett_mixer_elem_enum_info opt_gain = {
+	.start = 0,
+	.len = 2,
+	.offsets = {},
+	.names = (char const * const []){
+		"Lo", "Hi"
+	}
+};
+
 static const struct scarlett_mixer_elem_enum_info opt_impedance = {
 	.start = 0,
 	.len = 2,
@@ -652,8 +662,8 @@ static struct scarlett_device_info s6i6_info = {
 		{ .num = 1, .type = SCARLETT_SWITCH_PAD, .name = NULL},
 		{ .num = 2, .type = SCARLETT_SWITCH_IMPEDANCE, .name = NULL},
 		{ .num = 2, .type = SCARLETT_SWITCH_PAD, .name = NULL},
-		{ .num = 3, .type = SCARLETT_SWITCH_PAD, .name = NULL},
-		{ .num = 4, .type = SCARLETT_SWITCH_PAD, .name = NULL},
+		{ .num = 3, .type = SCARLETT_SWITCH_GAIN, .name = NULL},
+		{ .num = 4, .type = SCARLETT_SWITCH_GAIN, .name = NULL},
 	},
 
 	.matrix_mux_init = {
@@ -883,6 +893,15 @@ static int scarlett_controls_create_generic(struct usb_mixer_interface *mixer,
 			if (err < 0)
 				return err;
 			break;
+		case SCARLETT_SWITCH_GAIN:
+			sprintf(mx, "Input %d Gain Switch", ctl->num);
+			err = add_new_ctl(mixer, &usb_scarlett_ctl_enum,
+					  scarlett_ctl_enum_resume, 0x01,
+					  0x08, ctl->num, USB_MIXER_S16, 1, mx,
+					  &opt_gain, &elem);
+			if (err < 0)
+				return err;
+			break;
 		}
 	}