net: enetc: simplify enetc_num_stack_tx_queues()

We keep a pointer to the xdp_prog in the private netdev structure as
well; what's replicated per RX ring is done so just for more convenient
access from the NAPI poll procedure.

Simplify enetc_num_stack_tx_queues() by looking at priv->xdp_prog rather
than iterating through the information replicated per RX ring.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>