net: stmmac: preserve real_num_tx_queues on mqprio setup failure

With the FPE preemption-class mapping error now propagated from
stmmac_fpe_map_preemption_class(), tc_setup_dwmac510_mqprio() can fail
on the mapping step. The error path used to call stmmac_reset_tc_mqprio(),
which resets the number of real TX queues to priv->plat->tx_queues_to_use
(the platform maximum), overwriting the value that was active before the
offload was attempted (for example a lower count left over from a previous
mqprio configuration).

The issue can be triggered using the following configuration:

  # First mqprio config lowers the hw queue count below the platform
  # default (e.g. 8 TX queues).
  $tc qdisc add dev eth0 root handle 1: mqprio queues 2@0 2@2

  # Replace mqprio configuration with a second one that fails FPE
  # preemption-class mapping. stmmac driver resets the real_num_tx_queues
  # to the platform maximum, losing the previous configuration.
  $tc qdisc replace dev eth0 root handle 2: mqprio queues 2@0 2@2 fp E P

Save ndev->real_num_tx_queues before lowering it and restore it,
together with the TC-to-queue and priority-to-TC mappings, when the FPE
preemption-class mapping fails, instead of resetting the queue count to
the platform maximum.

Note that a failed setup makes the qdisc layer run mqprio_destroy() on
the new qdisc. Because priv->hw_offload is only assigned after
ndo_setup_tc() succeeds, mqprio_destroy() calls netdev_set_num_tc(dev, 0),
so dev->num_tc ends up 0 regardless of the driver-side restore and the
previous qdisc is not reactivated. The restore is still needed to keep
real_num_tx_queues and to avoid leaving the failed configuration's
TC-to-queue and priority-to-TC mappings in place.

Fixes: 195e4f409a40 ("net: stmmac: support fp parameter of tc-mqprio")
Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@oss.qualcomm.com>
Link: https://patch.msgid.link/20260911-stmmac-tc_setup_dwmac510_mqprio-error-path-v3-2-a76b1e2547c1@oss.qualcomm.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>