The NEC xHCI host controller firmware version can be found by putting a
vendor-specific command on the command ring and extracting the BCD
encoded-version out of the vendor-specific event TRB.
The firmware version debug line in dmesg will look like:
xhci_hcd 0000:05:00.0: NEC firmware version 30.21
(NEC merged with Renesas Technologies and became Renesas Electronics on
April 1, 2010. I have their OK to merge this vendor-specific code.)