net/i40e: fix dereference before null check

net/i40e: fix dereference before null check

Null-checking vsi suggests that it may be null, but it
has been dereferenced before null-checking. So move the
check to before the assignment statement using the pointer.

Coverity issue: 119265, 119266
Fixes: d0a349409bd7 ("i40e: support AQ based RSS config")
Fixes: 647d1eaf758b ("i40evf: support AQ based RSS config")
Signed-off-by: Beilei Xing <beilei.xing@intel.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>