powerpc/rtas: fix miswording in rtas_function kerneldoc

The 'filter' member is a pointer, not a bool; fix the wording
accordingly.
Signed-off-by: Nathan Lynch <nathanl@linux.ibm.com>
Reviewed-by: Andrew Donnellan <ajd@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230220-rtas-queue-for-6-4-v1-4-010e4416f13f@linux.ibm.com

arch/powerpc/kernel/rtas.c

@@ -68,7 +68,7 @@ struct rtas_filter {
  *                            functions are believed to have no users on
  *                            ppc64le, and we want to keep it that way. It does
  *                            not make sense for this to be set when @filter
- *                            is false.
+ *                            is NULL.
  */
 struct rtas_function {
 	s32 token;