crypto: akcipher - assume key is already set in maxsize

As of now, crypto_akcipher_maxsize() can not be reached without
successfully setting the key for the transformation. akcipher
algorithm implementations check if the key was set and then return
the output buffer size required for the given key.

Change the return type to unsigned int and always assume that this
function is called after a successful setkey of the transformation.
akcipher algorithm implementations will remove the check if key is not NULL
and directly return the max size.

Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>