net/sched: taprio: remove redundant FULL_OFFLOAD_IS_ENABLED check in taprio_enqueue

Since commit 13511704

 ("net: taprio offload: enforce qdisc to netdev
queue mapping"), __dev_queue_xmit() will select a txq->qdisc for the
full offload case of taprio which isn't the root taprio qdisc, so
qdisc enqueues will never pass through taprio_enqueue().

That commit already introduced one safety precaution check for
FULL_OFFLOAD_IS_ENABLED(); a second one is really not needed, so
simplify the conditional for entering into the GSO segmentation logic.
Also reword the comment a little, to appear more natural after the code
change.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>