TX: Use mac_hcksum_get() instead of hcksum_retrieve()

In the TX checksum offload code, use mac_hcksum_get() rather
than hcksum_retrieve().
hcksum_retrieve() is not a documented/published interface
RX: Use mac_hcksum_set() instead of hcksum_assoc()

In the RX checksum offload code, use mac_hcksum_set() rather
than hcksum_assoc().
hcksum_assoc() is not a documented/published interface

Also return different flags, since we are not passing the
calculated checksum as the 'value' argument.