i40e: point wb_desc at the nvm_wb_desc during i40e_read_nvm_aq

When introducing the functions to read the NVM through the AdminQ, we
did not correctly mark the wb_desc.

Fixes: 7073f46e

 ("i40e: Add AQ commands for NVM Update for X722", 2015-06-05)
Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>