net: ethernet: lantiq_etop: fix build errors/warnings

Fix build error and warnings reported by kernel test robot:

drivers/net/ethernet/lantiq_etop.c: In function 'ltq_etop_probe':
drivers/net/ethernet/lantiq_etop.c:673:15: error: implicit declaration of function 'device_property_read_u32' [-Werror=implicit-function-declaration]
     673 |         err = device_property_read_u32(&pdev->dev, "lantiq,tx-burst-length", &priv->tx_burst_len);

   drivers/net/ethernet/lantiq_etop.c: At top level:
   drivers/net/ethernet/lantiq_etop.c:730:1: warning: no previous prototype for 'init_ltq_etop' [-Wmissing-prototypes]
     730 | init_ltq_etop(void)

   drivers/net/ethernet/lantiq_etop.c: In function 'ltq_etop_hw_init':
   drivers/net/ethernet/lantiq_etop.c:276:25: warning: ignoring return value of 'request_irq' declared with attribute 'warn_unused_result' [-Wunused-result]
     276 |                         request_irq(irq, ltq_etop_dma_irq, 0, "etop_tx", priv);
   drivers/net/ethernet/lantiq_etop.c:284:25: warning: ignoring return value of 'request_irq' declared with attribute 'warn_unused_result' [-Wunused-result]
     284 |                         request_irq(irq, ltq_etop_dma_irq, 0, "etop_rx", priv);

Fixes: 14d4e308 ("net: lantiq: configure the burst length in ethernet drivers")
Fixes: dddb29e4 ("net: lantiq_etop: remove deprecated IRQF_DISABLED")
Fixes: 504d4721

 ("MIPS: Lantiq: Add ethernet driver")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reported-by: kernel test robot <lkp@intel.com>
Link: lore.kernel.org/r/202111090621.yjr9xuVj-lkp@intel.com
To: netdev@vger.kernel.org
Cc: Aleksander Jan Bajkowski <olek2@wp.pl>
Cc: Hauke Mehrtens <hauke@hauke-m.de>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: John Crispin <john@phrozen.org>
Cc: linux-mips@vger.kernel.org
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Michael Opdenacker <michael.opdenacker@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>