powerpc/perf/hv-24x7: Rename hv_24x7_event_update

For consistency with the pmu operation ->read() and with other
pmus, rename hv_24x7_event_update() to hv_24x7_event_read().

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