crypto: mediatek - don't return garbage err on successful return

In the case where keylen <= bs mtk_sha_setkey returns an uninitialized
return value in err.  Fix this by returning 0 instead of err.

Issue detected by static analysis with cppcheck.
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

drivers/crypto/mediatek/mtk-sha.c

@@ -878,7 +878,7 @@ static int mtk_sha_setkey(struct crypto_ahash *tfm,
 		bctx->opad[i] ^= 0x5c;
 	}
 
-	return err;
+	return 0;
 }
 
 static int mtk_sha_export(struct ahash_request *req, void *out)