USB: EHCI: add symbolic constants for QHs

This patch (as1568) introduces symbolic constants for some of the
less-frequently used bitfields in the QH structure.  This makes the
code a little easier to read and understand.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>