powerpc/rtasd: use correct OF API for event scan rate

rtas_token() should be used only for properties that are RTAS function
tokens. "rtas-event-scan-rate" does not contain a function token, but it
has the same size/format as token properties so reading it with
rtas_token() happens to work.

Convert to of_property_read_u32().

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://lore.kernel.org/r/20221118150751.469393-3-nathanl@linux.ibm.com