i40iw: Correct ARP index mask

The ARP table entry indexes are aliased to 12bits
instead of the intended 16bits when uploaded to
the QP Context. This will present an issue when the
number of connections exceeds 4096 as ARP entries are
reused. Fix this by adjusting the mask to account for
the full 16bits.

Fixes: 4e9042e6 ("i40iw: add hw and utils files")
Signed-off-by: Mustafa Ismail <mustafa.ismail@intel.com>
Signed-off-by: Shiraz Saleem <shiraz.saleem@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>