xhci driver displays the supported xHC USB revision in a message during
driver load:

"Host supports USB 3.1 Enhanced SuperSpeed"

Get the USB minor revision number from the xhci protocol capability.
This will show the correct supported revisions for new USB 3.2 and later
hosts

Don't rely on the SBRN (serial bus revision number) register, it's often
showing 0x30 (USB3.0) for hosts that support USB 3.1
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>