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