powerpc/pseries: fix plpks_read_var() code for different consumers

Even though plpks_read_var() is currently called to read variables
owned by different consumers, it internally supports only OS consumer.

Fix plpks_read_var() to handle different consumers correctly.

Fixes: 2454a7af ("powerpc/pseries: define driver for Platform KeyStore")
Signed-off-by: Nayna Jain <nayna@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20221106205839.600442-7-nayna@linux.ibm.com