powerpc/pseries/lparcfg: Fix printing of system_active_processors

When printing the information "system_active_processors", the variable
partition_potential_processors is used instead of
partition_active_processors. The wrong value is displayed.

Use partition_active_processors instead.

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20241010-vdso-generic-base-v1-23-b64f0842d512@linutronix.de