powerpc/pseries/hv-24x7: convert to papr_sysparm API

The new papr_sysparm API handles the details of system parameter
retrieval. Use that instead of open-coding the RTAS call, work area
management, and retries.

Signed-off-by: Nathan Lynch <nathanl@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20230125-b4-powerpc-rtas-queue-v3-17-26929c8cce78@linux.ibm.com