net: stmmac: do not clear TBS enable bit on link up/down

With the dma conf being reallocated on each call to stmmac_open(), any
information in there is lost, unless we specifically handle it.

The STMMAC_TBS_EN bit is set when adding an etf qdisc, and the etf qdisc
therefore would stop working when link was set down and then back up.

Fixes: ba39b344 ("net: ethernet: stmicro: stmmac: generate stmmac dma conf before open")
Cc: stable@vger.kernel.org
Signed-off-by: Esben Haabendal <esben@geanix.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>