crypto: lz4 - fixed decompress function to return error code

Decompress function in LZ4 library is supposed to return an error code or
negative result. But, it returns -1 when any error is detected. Return
error code when the library returns negative value.

Signed-off-by: Myungho Jung <mhjungk@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>