powerpc/pseries: replace kmalloc with kzalloc in PLPKS driver

Replace kmalloc with kzalloc in construct_auth() function to default
initialize structure with zeroes.

Signed-off-by: Nayna Jain <nayna@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20221106205839.600442-6-nayna@linux.ibm.com