usb: ch9: Add size macro for SSP dev cap descriptor

The SuperspeedPlus Device Capability Descriptor has a variable size
depending on the number of sublink speed attributes.

This patch adds a macro to calculate that size. The macro takes one
argument, the Sublink Speed Attribute Count (SSAC) as reported by the
descriptor in bmAttributes[4:0].

See USB 3.1 9.6.2.5, Table 9-19.

Signed-off-by: John Youn <johnyoun@synopsys.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>

include/uapi/linux/usb/ch9.h

@@ -923,6 +923,12 @@ struct usb_ptm_cap_descriptor {
 	__u8  bDevCapabilityType;
 } __attribute__((packed));
 
+/*
+ * The size of the descriptor for the Sublink Speed Attribute Count
+ * (SSAC) specified in bmAttributes[4:0].
+ */
+#define USB_DT_USB_SSP_CAP_SIZE(ssac)	(16 + ssac * 4)
+
 /*-------------------------------------------------------------------------*/
 
 /* USB_DT_WIRELESS_ENDPOINT_COMP:  companion descriptor associated with