staging: comedi: don't expose comedi_device_{attach,detach}

These functions are only used by the comedi core. Move the
prototypes to comedi_internal.h so they are not exposed to
the comedi drivers.

Tidy up comedi_internal.h a bit so that all the internal stuff
in drivers.c is grouped.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>