ip6erspan: remove the incorrect mtu limit for ip6erspan

ip6erspan driver calls ether_setup(), after commit 61e84623
("net: centralize net_device min/max MTU checking"), the range
of mtu is [min_mtu, max_mtu], which is [68, 1500] by default.

It causes the dev mtu of the erspan device to not be greater
than 1500, this limit value is not correct for ip6erspan tap
device.

Fixes: 61e84623 ("net: centralize net_device min/max MTU checking")
Signed-off-by: Haishuang Yan <yanhaishuang@cmss.chinamobile.com>
Acked-by: William Tu <u9012063@gmail.com>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>