crypto: arc4 - convert to skcipher API

Convert the "ecb(arc4)" algorithm from the deprecated "blkcipher" API to
the "skcipher" API.

(Note that this is really a stream cipher and not a block cipher in ECB
mode as the name implies, but that's a problem for another day...)

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