staging: comedi: tidy up the general purpose driver functions

Group all the general comedi driver register/config/attach
prototypes into one place in comedidev.h.

Reorder the functions in drivers.c a bit so they are in a more
logical usage order (bottom to top).

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>