esp6: fix incorrect null pointer check on xo

The check for xo being null is incorrect, currently it is checking
for non-null, it should be checking for null.

Detected with CoverityScan, CID#1429349 ("Dereference after null check")

Fixes: 7862b405 ("esp: Add gso handlers for esp4 and esp6")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>