crypto: atmel-aes - Change data type for "lastc" buffer

In case of in-place decryption, the "lastc" buffer is used to copy
the last ciphertext block before the decryption of the message. It
is later used to update the req->iv of the skcipher_request.

"lastc" variable is not used to interact with the hardware, there
is no restriction to be of type "u32". Change the type of "lastc"
to "u8".

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