powerpc/pseries: Use correct event modifier in rtas_parse_epow_errlog()

rtas_parse_epow_errlog() should pass 'modifier' to
handle_system_shutdown, because event modifier only use
bottom 4 bits.

Reviewed-by: Tyrel Datwyler <tyreld@linux.ibm.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20191023134838.21280-1-yuehaibing@huawei.com