net: stmmac: Always arm TX Timer at end of transmission start

If TX Coalesce timer is enabled we should always arm it, otherwise we
may hit the case where an interrupt is missed and the TX Queue will
timeout.

Arming the timer does not necessarly mean it will run the tx_clean()
because this function is wrapped around NAPI launcher.

Fixes: 9125cdd1 ("stmmac: add the initial tx coalesce schema")
Signed-off-by: Jose Abreu <Jose.Abreu@synopsys.com>
Signed-off-by: David S. Miller <davem@davemloft.net>