When disable CBS, mode_to_use parameter is not updated even the operation
mode of Tx Queue is changed to Data Centre Bridging (DCB). Therefore,
when tc_setup_cbs() function is called to re-enable CBS, the operation
mode of Tx Queue remains at DCB, which causing CBS fails to work.

This patch updates the value of mode_to_use parameter to MTL_QUEUE_DCB
after operation mode of Tx Queue is changed to DCB in stmmac_dma_qmode()
callback function.

Fixes: 1f705bc6 ("net: stmmac: Add support for CBS QDISC")
Suggested-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>
Signed-off-by: Mohammad Athari Bin Ismail <mohammad.athari.ismail@intel.com>
Signed-off-by: Song, Yoong Siang <yoong.siang.song@intel.com>
Reviewed-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Acked-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>
Link: https://lore.kernel.org/r/1612447396-20351-1-git-send-email-yoong.siang.song@intel.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>