powerpc/perf/hv-24x7: Simplify extracting counter from result buffer

Simplify code that extracts a 24x7 counter from the HCALL's result buffer.

Suggested-by: Joe Perches <joe@perches.com>
Signed-off-by: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>