usb: ehci: only prepare zero packet for out transfer if required

Obviously, ZLP is only required for transfer of OUT direction,
so just take same policy with UHCI for ZLP packet.

Signed-off-by: Ming Lei <tom.leiming@gmail.com>
Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>