powerpc/pseries: Clarify warning when PLPKS password already set

When the H_PKS_GEN_PASSWORD hcall returns H_IN_USE, operations that require
authentication (i.e. anything other than reading a world-readable variable)
will not work.

The current error message doesn't explain this clearly enough. Reword it
to emphasise that authenticated operations will fail.

Signed-off-by: Andrew Donnellan <ajd@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20230210080401.345462-22-ajd@linux.ibm.com