[SeaBIOS] seabios does not support booting from usb-uas with nec-usb-xhci

[Tom Yan]

Strangely, seabios supports booting from usb-uas with usb-ehci:

https://drive.google.com/open?id=0B50PCxfm5KU1dUFTUG9HU0pEcXc

and usb-bot with nec-usb-xhci:

https://drive.google.com/open?id=0B50PCxfm5KU1QnVzdTBIdEFzZW8

but not usb-uas with nec-usb-xhci:

https://drive.google.com/open?id=0B50PCxfm5KU1Z0t1akNXaVZZLTQ

This makes me wonder if it is just a missing feature of a bug.

Btw, not sure if it's relevant, booting usb-uas is not at all
supported in ovmf: https://github.com/tianocore/edk2/issues/85