USB: minor ehci xITD simplifications

Remove two (or one) conditional tests in per-urb isochronous
transfer setup code paths.

Signed-off-by: Karsten Wiese <fzu@wemgehoertderstaat.de>
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>