Staging: comedi: make comedi_alloc_board_minor local to comedi core

No one outside of the comedi core calls this function, so create
an internal.h file to put the prototype in, and don't export
it to the world.

Cc: Ian Abbott <abbotti@mev.co.uk>
Cc: Frank Mori Hess <fmhess@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>