staging: comedi: move comedi_alloc_subdevice_minor declarations

Move the declarations of `comedi_alloc_subdevice_minor()` and
`comedi_free_subdevice_minor()` from "comedidev.h" to
"comedi_internal.h" since they are only of interest to the comedi core,
and are not exported to the low-level comedi drivers.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>