staging/rtl8192u: switch to RC4 library interface

Switch to the ARC4 library interface, to remove the pointless
dependency on the skcipher API, from which we will hopefully be
able to drop ecb(arc4) skcipher support.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>