i40iw: Fix error handling in i40iw_manage_arp_cache()

The i40iw_arp_table() function can return -EOVERFLOW if
i40iw_alloc_resource() fails so we can't just test for "== -1".

Fixes: 4e9042e6 ("i40iw: add hw and utils files")
Link: https://lore.kernel.org/r/20200422092211.GA195357@mwanda


Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Shiraz Saleem <shiraz.saleem@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>