crypto: atmel-ecc - fix signed integer to u8 assignment

static checker warning:
        drivers/crypto/atmel-ecc.c:281 atmel_ecdh_done()
        warn: assigning (-22) to unsigned variable 'status'

Similar warning can be raised in atmel_ecc_work_handler()
when atmel_ecc_send_receive() returns an error. Fix this too.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>