i40e: Fix incorrect address type for IPv6 flow rules

It was not possible to create 1-tuple flow director
rule for IPv6 flow type. It was caused by incorrectly
checking for source IP address when validating user provided
destination IP address.

Fix this by changing ip6src to correct ip6dst address
in destination IP address validation for IPv6 flow type.

Fixes: efca91e8 ("i40e: Add flow director support for IPv6")
Signed-off-by: Sylwester Dziedziuch <sylwesterx.dziedziuch@intel.com>
Tested-by: Gurucharan <gurucharanx.g@intel.com> (A Contingent worker at Intel)
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>