cfg80211/mac80211: assume CHECKSUM_COMPLETE includes SNAP

cfg80211/mac80211: assume CHECKSUM_COMPLETE includes SNAP

There's currently only one driver that reports CHECKSUM_COMPLETE,
that is iwlwifi. The current hardware there calculates checksum
after the SNAP header, but only RFC 1042 (and some other cases,
but replicating the exact hardware logic for corner cases in the
driver seemed awkward.)

Newer generations of hardware will checksum _including_ the SNAP,
which makes things easier.

To handle that, simply always assume the checksum _includes_ the
SNAP header, which this patch does, requiring to first add it
for older iwlwifi hardware, and then remove it again later on
conversion.

Alternatively, we could have:

 1) Always assumed the checksum starts _after_ the SNAP header;
    the problem with this is that we'd have to replace the exact
    "what is the SNAP" check in iwlwifi that cfg80211 has.

 2) Made it configurable with some flag, but that seemed like too
    much complexity.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://lore.kernel.org/r/iwlwifi.20220202104617.230736e19e0e.I3e6745873585ad943c152fab9e23b5221f17a95f@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>