iwlwifi: mvm: report checksum is done also for IPv6 packets

Currently the code checks if hardware reported both L4 and L3
checksums as valid, and only then reports it as validated to
the stack.
However, IPv6 does not have checksum at all and the L3 checksum
valid bit is always off for IPv6 packets, with the result of the
stack re-validating L4 checksum.
Fix code to set CHECKSUM_UNNECESSARY also for IPv6 packets whose
TCP/UDP checksum was verified.

Signed-off-by: Sara Sharon <sara.sharon@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>