staging: comedi: tests: Correct unittest_fptr

staging: comedi: tests: Correct unittest_fptr

The definition of the `unittest_fptr` function pointer type has the
wrong function return type `void *` instead of `void`.  The problem is
hidden by a bunch of unnecessary type-casts.  Fix the type definition
and remove the type-casts.

Cc: Spencer E. Olson <olsonse@umich.edu>
Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Link: https://lore.kernel.org/r/20210407140142.447250-6-abbotti@mev.co.uk
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>