net: enetc: add graceful stop to safely reinitialize the TX Ring

For ENETC v4, the PIR and CIR will be reset if they are not equal when
reinitializing the TX BD ring. However, resetting the PIR and CIR alone
is insufficient. When a link-down event occurs while the TX BD ring is
transmitting frames, subsequent reinitialization of the TX BD ring may
cause it to malfunction. For example, the below steps can reproduce the
problem.

1. Unplug the cable when the TX BD ring is busy transmitting frames.
2. Disable the network interface (ifconfig eth0 down).
3. Re-enable the network interface (ifconfig eth0 up).
4. Plug in the cable, the TX BD ring may fail to transmit packets.

When the link-down event occurs, enetc4_pl_mac_link_down() only clears
PMa_COMMAND_CONFIG[TX_EN] to disable MAC transmit data path. It doesn't
set PORT[TXDIS] to 1 to flush the TX BD ring. Therefore, reinitializing
the TX BD ring at this point is unsafe. To safely reinitialize the TX BD
ring after a link-down event, we checked with the NETC IP team, a proper
Ethernet MAC graceful stop is necessary. Therefore, add the Ethernet MAC
graceful stop to the link-down event handler enetc4_pl_mac_link_down().

Fixes: 99100d0d9922 ("net: enetc: add preliminary support for i.MX95 ENETC PF")
Signed-off-by: Wei Fang <wei.fang@nxp.com>
Reviewed-by: Claudiu Manoil <claudiu.manoil@nxp.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20260324062121.2745033-3-wei.fang@nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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