i40iw: Fix typecast of tcp_seq_num

The typecast of tcp_seq_num incorrectly uses u8. Fix by
casting to u32.

Signed-off-by: Mustafa Ismail <mustafa.ismail@intel.com>
Signed-off-by: Henry Orosco <henry.orosco@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>