The EHCI driver was never adjusting the full speed maximum packet size up
(when enumerating through a transaction translating hub).  This broke the
enumeration of some devices (maxpacket != 8) pretty early.

This patch updates EHCI to fix the bug, and does minor cleanup to usbcore
logic that figures out ep0 maxpacket.  I left the partial read in for all
speeds, even though only full speed needs it.