powerpc: Make set_rtc_time() return error code from lower-level function

powerpc: Make set_rtc_time() return error code from lower-level function

Previously it ignored the return value from ppc_md.set_rtc_time,
but in fact the functions that that can point to do return a
useful error code, so return it from set_rtc_time().

Signed-off-by: Paul Mackerras <paulus@samba.org>