bnxt_en: bring back rtnl lock in bnxt_shutdown

Taehee reports missing rtnl from bnxt_shutdown path:

inetdev_event (./include/linux/inetdevice.h:256 net/ipv4/devinet.c:1585)
notifier_call_chain (kernel/notifier.c:85)
__dev_close_many (net/core/dev.c:1732 (discriminator 3))
kernel/locking/mutex.c:713 kernel/locking/mutex.c:732)
dev_close_many (net/core/dev.c:1786)
netif_close (./include/linux/list.h:124 ./include/linux/list.h:215
bnxt_shutdown (drivers/net/ethernet/broadcom/bnxt/bnxt.c:16707) bnxt_en
pci_device_shutdown (drivers/pci/pci-driver.c:511)
device_shutdown (drivers/base/core.c:4820)
kernel_restart (kernel/reboot.c:271 kernel/reboot.c:285)

Bring back the rtnl lock.

Link: https://lore.kernel.org/netdev/CAMArcTV4P8PFsc6O2tSgzRno050DzafgqkLA2b7t=Fv_SY=brw@mail.gmail.com/
Fixes: 004b500 ("eth: bnxt: remove most dependencies on RTNL")
Reported-by: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: Stanislav Fomichev <sdf@fomichev.me>
Tested-by: Taehee Yoo <ap420073@gmail.com>
Tested-by: Breno Leitao <leitao@debian.org>
Link: https://patch.msgid.link/20250328174216.3513079-1-sdf@fomichev.me
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: Stanislav Fomichev

drivers/net/ethernet/broadcom/bnxt/bnxt.c

@@ -16698,6 +16698,7 @@ static void bnxt_shutdown(struct pci_dev *pdev)
 	if (!dev)
 		return;
 
+	rtnl_lock();
 	netdev_lock(dev);
 	bp = netdev_priv(dev);
 	if (!bp)
@@ -16717,6 +16718,7 @@ static void bnxt_shutdown(struct pci_dev *pdev)
 
 shutdown_exit:
 	netdev_unlock(dev);
+	rtnl_unlock();
 }
 
 #ifdef CONFIG_PM_SLEEP