powerpc/secvar: Warn and error if multiple secvar ops are set

The secvar code only supports one consumer at a time.

Multiple consumers aren't possible at this point in time, but we'd want
it to be obvious if it ever could happen.

Signed-off-by: Russell Currey <ruscur@russell.cc>
Co-developed-by: Andrew Donnellan <ajd@linux.ibm.com>
Signed-off-by: Andrew Donnellan <ajd@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20230210080401.345462-6-ajd@linux.ibm.com