crypto: skcipher - call cond_resched() directly

In skcipher_walk_done(), instead of calling crypto_yield() which
requires a translation between flags, just call cond_resched() directly.
This has the same effect.

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