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

In the Linux kernel, the following vulnerability has been resolved: net: stmmac: Fix zero-division error when disabling tc cbs The commit b8c43360f6e4 ("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.