plugin.c: -Wpendantic cast fixes for callback handling

plugin.c: -Wpendantic cast fixes for callback handling

Remove void* _data_ pointer casts with correct function pointer types
and casts.  In some cases this allowed dropping the cast completely,
as I used same type for "cf_callback" member as the simplest init()
and shutdown() callbacks.

Only remaining pedantic warning is unfixable, as dlsym() API returns
void* data pointer, instead of function pointer.

Signed-off-by: Eero Tamminen <eero.t.tamminen@intel.com>