compress/zlib: fix error handling

Add missing return after setting the error status in case of
invalid flush_flag in the operation.
The issue was found by the coverity scan as the fin_flush variable,
not initialized in such case, was used later in the flow.

Coverity issue: 340859
Fixes: c7b436ec95fd ("compress/zlib: support burst enqueue/dequeue")
Cc: stable@dpdk.org

Signed-off-by: Adam Dybkowski <adamx.dybkowski@intel.com>

drivers/compress/zlib/zlib_pmd.c

@@ -30,6 +30,7 @@ process_zlib_deflate(struct rte_comp_op *op, z_stream *strm)
 	default:
 		op->status = RTE_COMP_OP_STATUS_INVALID_ARGS;
 		ZLIB_PMD_ERR("Invalid flush value\n");
+		return;
 	}
 
 	if (unlikely(!strm)) {