commit be27b896 ("net: stmmac: replace priv->speed with
the portTransmitRate from the tc-cbs parameters") introduced
a problem. When deleting, it prompts "Invalid portTransmitRate
0 (idleSlope - sendSlope)" and exits. Add judgment on cbs.enable.
Only when offload is enabled, speed divider needs to be calculated.

Fixes: be27b896 ("net: stmmac: replace priv->speed with the portTransmitRate from the tc-cbs parameters")
Signed-off-by: Xiaolei Wang <xiaolei.wang@windriver.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://lore.kernel.org/r/20240617013922.1035854-1-xiaolei.wang@windriver.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>