powerpc/pseries/dlpar: use rtas_get_sensor()

Instead of making bare calls to get-sensor-state, use
rtas_get_sensor(), which correctly handles busy and extended delay
statuses.

Fixes: ab519a01

 ("powerpc/pseries: Kernel DLPAR Infrastructure")
Signed-off-by: Nathan Lynch <nathanl@linux.ibm.com>
Reviewed-by: Laurent Dufour <ldufour@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210504025329.1713878-1-nathanl@linux.ibm.com