net: stmmac: stop each tx channel independently

If clear GMAC_CONFIG_TE bit, it would stop all tx channels, but users
may only want to stop specific tx channel.

Fixes: 48863ce5 ("stmmac: add DMA support for GMAC 4.xx")
Signed-off-by: Joakim Zhang <qiangqing.zhang@nxp.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>