i40e: avoid potential null pointer dereference when assigning len

There is a sanitcy check for desc being null in the first line of
function i40evf_debug_aq.  However, before that, aq_desc is cast from
desc, and aq_desc is being dereferenced on the assignment of len, so
this could be a potential null pointer deference.  Fix this by moving
the initialization of len to the code block where len is being used
and hence at this point we know it is OK to dereference aq_desc.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>