thunderbolt: Clarify register definitions for `tb_cap_plug_events`

The USB4 1.0 specification outlines the `cap_plug_events` structure as
`VSC_CS_1`.  This shows that 4 bits of `VSC_CS_1` are TBT3 compatible in
USB4, but TBT3 controllers also support disabling XHCI.

Update the names and comments to more closely match the specification.
This should not change anything functionally.

Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>