iwlwifi: mvm: use a define for checksum flags mask

iwlwifi: mvm: use a define for checksum flags mask

For the upcoming Bz devices, we will set NETIF_F_HW_CSUM instead
of NETIF_F_IP_CSUM and NETIF_F_IPV6_CSUM. However, we still need
to be able to remove all the checksum bits, so add a mask for the
removal, and keep the old define for the feature advertisement.

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.20211219121514.f05488ce8b83.I65bb83721498d8433e4ee2b09415eb74ab579445@changeid
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>