lwtunnel: change to use nla_put_u8 for LWTUNNEL_IP_OPT_ERSPAN_VER

LWTUNNEL_IP_OPT_ERSPAN_VER is u8 type, and nla_put_u8 should have
been used instead of nla_put_u32(). This is a copy-paste error.

Fixes: b0a21810 ("lwtunnel: add options setting and dumping for erspan")
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Reviewed-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>