crypto: sahara - avoid skcipher fallback code duplication

Factor out duplicated skcipher fallback handling code to a helper function
sahara_aes_fallback(). Also, keep a single check if fallback is required in
sahara_aes_crypt().

Signed-off-by: Ovidiu Panait <ovidiu.panait@windriver.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>