When testing streaming input (or streaming output) command functionality
for a subdevice, attempt to make it the current 'read' (or 'write')
subdevice by calling the new 'comedi_set_read_subdevice()' (or
'comedi_set_write_subdevice()') function.
That is currently only supported by the Linux "in-tree" version of
Comedi, since kernel version 3.19. If the attempt fails, and the
subdevice is not the current 'read' (or 'write') subdevice, the test
will report "not applicable" as before.
