crypto: powerpc - convert to use crypto_simd_usable()

Replace all calls to in_interrupt() in the PowerPC crypto code with
!crypto_simd_usable().  This causes the crypto self-tests to test the
no-SIMD code paths when CONFIG_CRYPTO_MANAGER_EXTRA_TESTS=y.

The p8_ghash algorithm is currently failing and needs to be fixed, as it
produces the wrong digest when no-SIMD updates are mixed with SIMD ones.

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