wil6210: fix potential null dereference of ndev before null check

The pointer ndev is being dereferenced before it is being null checked,
hence there is a potential null pointer deference. Fix this by only
dereferencing ndev after it has been null checked

Detected by CoverityScan, CID#1467010 ("Dereference before null check")

Fixes: e00243fa ("wil6210: infrastructure for multiple virtual interfaces")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Maya Erez <merez@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>