powerpc/pseries: Hypervisor call tracepoints hcall_stats touchup

The tb_total and purr_total values reported via the hcall_stats code
should be cumulative, rather than being replaced by the latest delta tb
or purr value.

Tested-by: Will Schmidt <will_schmidt@vnet.ibm.com>
Signed-off-by: Will Schmidt <will_schmidt@vnet.ibm.com>
Acked-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>