net: stmmac: Fix zero-division error when disabling tc cbs

jira NONE_AUTOMATION
cve CVE-2024-49977
Rebuild_History Non-Buildable kernel-5.14.0-570.12.1.el9_6
commit-author KhaiWenTan <khai.wen.tan@linux.intel.com>
commit 675faf5

The commit b8c4336 ("net: stmmac: No need to calculate speed divider
when offload is disabled") allows the "port_transmit_rate_kbps" to be
set to a value of 0, which is then passed to the "div_s64" function when
tc-cbs is disabled. This leads to a zero-division error.

When tc-cbs is disabled, the idleslope, sendslope, and credit values the
credit values are not required to be configured. Therefore, adding a return
statement after setting the txQ mode to DCB when tc-cbs is disabled would
prevent a zero-division error.

Fixes: b8c4336 ("net: stmmac: No need to calculate speed divider when offload is disabled")
	Cc: <stable@vger.kernel.org>
Co-developed-by: Choong Yong Liang <yong.liang.choong@linux.intel.com>
	Signed-off-by: Choong Yong Liang <yong.liang.choong@linux.intel.com>
	Signed-off-by: KhaiWenTan <khai.wen.tan@linux.intel.com>
	Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20240918061422.1589662-1-khai.wen.tan@linux.intel.com
	Signed-off-by: Paolo Abeni <pabeni@redhat.com>

(cherry picked from commit 675faf5)
	Signed-off-by: Jonathan Maple <jmaple@ciq.com>

Author: PlaidCat

drivers/net/ethernet/stmicro/stmmac/stmmac_tc.c

@@ -396,6 +396,7 @@ static int tc_setup_cbs(struct stmmac_priv *priv,
 			return ret;
 
 		priv->plat->tx_queues_cfg[queue].mode_to_use = MTL_QUEUE_DCB;
+		return 0;
 	}
 
 	/* Final adjustments for HW */