iwlwifi: mvm: properly enable IP header checksumming

The code was intended to enable IP header checksumming on AMSDUs, but
failed to really do so because the A-MSDU bit was set after all the
checksumming bits, and thus checking for A-MSDU could never be true.

Fix this by setting the A-MSDU bit before the offload bits.

Fixes: 5e6a98dc ("iwlwifi: mvm: enable TCP/UDP checksum support for 9000 family")
Reported-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>