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