SeaBIOS USB fixes

[Kevin O'Connor]

Hi,

The current SeaBIOS git master branch now has some important fixes for
handling USB drives.  These fixes have improved the stability of both
UHCI and EHCI controllers.  If you were seeing issues booting from USB
with SeaBIOS, please retry with the latest SeaBIOS git.

If you continue to see failures, please capture the output of the
serial log and send it to the SeaBIOS mailing list.

Thanks,
-Kevin