crypto: sun4i-ss - use GENMASK to generate masks

Use the GENMASK helper instead of custom calculations to generate masks,
It also helps the readability.

Signed-off-by: Antoine Tenart <antoine.tenart@free-electrons.com>
Tested-by: Corentin Labbe <clabbe.montjoie@gmail.com>
Acked-by: Corentin Labbe <clabbe.montjoie@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

drivers/crypto/sunxi-ss/sun4i-ss-hash.c

@@ -384,11 +384,14 @@ static int sun4i_hash(struct ahash_request *areq)
 			writesl(ss->base + SS_RXFIFO, op->buf, nwait);
 			op->byte_count += 4 * nwait;
 		}
+
 		nbw = op->len - 4 * nwait;
-		wb = *(u32 *)(op->buf + nwait * 4);
-		wb &= (0xFFFFFFFF >> (4 - nbw) * 8);
+		if (nbw) {
+			wb = *(u32 *)(op->buf + nwait * 4);
+			wb &= GENMASK((nbw * 8) - 1, 0);
 
-		op->byte_count += nbw;
+			op->byte_count += nbw;
+		}
 	}
 
 	/* write the remaining bytes of the nbw buffer */