i40iw: Account for IPv6 header when setting MSS

The IPv6 header size is not subtracted from MTU when MSS is
set for QPs.

Save MTU opposed to MSS in the vsi struct during
initialization and calculate the MSS based on IPv4 vs
IPv6 connection.

Fixes: f27b4746 ("i40iw: add connection management code")
Signed-off-by: Shiraz Saleem <shiraz.saleem@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>