net: ethernet: mtk_eth_soc: do not overwrite mtu configuration running reset routine

Restore user configured MTU running mtk_hw_init() during tx timeout routine
since it will be overwritten after a hw reset.

Reported-by: Felix Fietkau <nbd@nbd.name>
Fixes: 9ea4d311 ("net: ethernet: mediatek: add the whole ethernet reset into the reset process")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>