net: systemport: Remove adaptive TX coalescing

Adaptive TX coalescing is not currently giving us any advantages and
ends up making the CPU spin more frequently until TX completion. Deny
and disable adaptive TX coalescing for now and rely on static
configuration, we can always add it back later.

Reviewed-by: Tal Gilboa <talgi@mellanox.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>