usb: audio-v2: add ability to define feature unit descriptor

Similar to UAC1 spec, UAC2 feature unit descriptor
has variable size.

Current audio-v2 feature unit descriptor structure
is used for parsing descriptors, but can't be used
to define your own descriptor.

Add a new macro similar to what audio v1 already has.
Signed-off-by: Ruslan Bilovol <ruslan.bilovol@gmail.com>
Signed-off-by: Pavel Hofman <pavel.hofman@ivitera.com>
Link: https://lore.kernel.org/r/20210712125529.76070-2-pavel.hofman@ivitera.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

include/linux/usb/audio-v2.h

@@ -156,6 +156,20 @@ struct uac2_feature_unit_descriptor {
 	__u8 bmaControls[]; /* variable length */
 } __attribute__((packed));
 
+#define UAC2_DT_FEATURE_UNIT_SIZE(ch)		(6 + ((ch) + 1) * 4)
+
+/* As above, but more useful for defining your own descriptors: */
+#define DECLARE_UAC2_FEATURE_UNIT_DESCRIPTOR(ch)		\
+struct uac2_feature_unit_descriptor_##ch {			\
+	__u8  bLength;						\
+	__u8  bDescriptorType;					\
+	__u8  bDescriptorSubtype;				\
+	__u8  bUnitID;						\
+	__u8  bSourceID;					\
+	__le32 bmaControls[ch + 1];				\
+	__u8  iFeature;						\
+} __packed
+
 /* 4.7.2.10 Effect Unit Descriptor */
 
 struct uac2_effect_unit_descriptor {