powerpc/powernv: handle OPAL_SUCCESS return in opal_sensor_read

powerpc/powernv: handle OPAL_SUCCESS return in opal_sensor_read

Currently, when a sensor value is read, the kernel calls OPAL, which in
turn builds a message for the FSP, and waits for a message back.

The new device tree for OPAL sensors [1] adds new sensors that can be
read synchronously (core temperatures for instance) and that don't need
to wait for a response.

This patch modifies the opal call to accept an OPAL_SUCCESS return value
and cover the case above.

[1] https://lists.ozlabs.org/pipermail/skiboot/2015-March/000639.html

Signed-off-by: Cédric Le Goater <clg@fr.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>