i40iw: fix null pointer dereference on a null wqe pointer

Currently the null check for wqe is incorrect and lets a null wqe
be passed to set_64bit_val and this indexes into the null pointer
causing a null pointer dereference.  Fix this by fixing the null
pointer check to return an error if wqe is null.

Link: https://lore.kernel.org/r/20200401224921.405279-1-colin.king@canonical.com
Addresses-Coverity: ("dereference after a null check")
Fixes: 4b34e23f

 ("i40iw: Report correct firmware version")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Shiraz Saleem <shiraz.saleem@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>