lan78xx: Fix boot-time RTNL warning #4302

Job	Run time
build (arm64, arm64, defconfig, kernel8)	23m 46s
build (bcmrpi, arm, bcmrpi_defconfig, kernel)	19m 7s
build (bcm2709, arm, bcm2709_defconfig, kernel7)	20m 1s
build (bcm2835, arm, bcm2835_defconfig, kernel)	6m 34s
build (bcm2712, arm64, bcm2712_defconfig, kernel_2712)	25m 45s
build (bcm2711, arm64, bcm2711_defconfig, kernel8)	26m 34s
build (bcm2711_rt, arm64, bcm2711_rt_defconfig, kernel8_rt)	26m 56s
	2h 28m 43s

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