Fixes the donehead changed before ack bug in usbohci by noting that donehead
doesn't really change, just Wdh is not set and it's inappropriate to be doing
anything with donehead.

Also, disabling ohc interrupts during that routine (which presumably prevents
the TD structure from changing. And a misunderstanding fix of how
ohci->intrenable and ohci->interdisable work.

Note, I don't have a pc that has this bug, just the OLPC.