Attempt to make the streaming command demos work if the specified
subdevice is not the default 'read' or 'write' subdevice (depending on
the direction of the command).
This uses the new comedi_set_read_subdevice() and
comedi_set_write_subdevice() functions.
Changing the read or write subdevice is currently supported only by the
Linux "in-tree" version of Comedi since kernel version 3.19.
