qcacld-3.0: Initialize local variable old_credit before using it

In some platform, it reports error "old_credit is used uninitialized".
Initialize old_credit before using it.

Change-Id: I06351bba0abdfc5efb32406d1d245f8d8c658684
CRs-Fixed: 2495209

core/dp/txrx/ol_tx_send.c

@@ -1036,7 +1036,8 @@ void ol_tx_dump_group_credit_stats(ol_txrx_pdev_handle pdev)
 {
 	uint16_t i, j, is_break = 0;
 	int16_t curr_index, old_index, wrap_around;
-	uint16_t curr_credit, old_credit, mem_vdevs;
+	uint16_t curr_credit, mem_vdevs;
+	uint16_t old_credit = 0;
 
 	txrx_nofl_info("Group credit stats:");
 	txrx_nofl_info("  No: GrpID: Credit: Change: vdev_map");