powerpc/security: change link stack flush state to the flush type enum

Prepare to allow for hardware link stack flushing by using the
none/sw/hw type, same as the count cache state.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200609070610.846703-3-npiggin@gmail.com