esp6: remove a duplicative condition

Fixes coccicheck warnings:
./net/ipv6/esp6_offload.c:319:32-34:
WARNING !A || A && B is equivalent to !A || B

Signed-off-by: Junlin Yang <yangjunlin@yulong.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>