i40e: avoid null pointer dereference

In function i40e_debug_aq parameter desc is assumed to be
possibly NULL. Do not dereference it before checking the
value.

Fixes: f905dd62 ("i40e/i40evf: add max buf len to aq debug print helper")
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>