ip6_gre: remove redundant 'tunnel' setting in ip6erspan_tap_init()

'tunnel' was already set at the start of ip6erspan_tap_init().

Fixes: 5a963eb6

 ("ip6_gre: Add ERSPAN native tunnel support")
Signed-off-by: Alexey Kodanev <alexey.kodanev@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>