crypto: powerpc - don't unnecessarily use atomic scatterwalk

The PowerPC SPE implementations of AES modes only disable preemption
during the actual encryption/decryption, not during the scatterwalk
functions.  It's therefore unnecessary to request an atomic scatterwalk.
So don't do so.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>