gianfar: Fix Tx csum generation errata handling

Both [eTSEC76] and [eTSEC12] errata relate to Tx checksum generation
(for some MPC83xx and MCP8548 older revisions). They require the same
workaround: manual checksum computation and insertion, and disabling
the H/W Tx csum acceleration feature (per frame) through Tx FCB
(Frame Control Block) csum offload settings.

The workaround for [eTSEC76] needs to be fixed because it currently
fails to disable H/W Tx csum insertion via FCB. This patch fixes it
and provides a common workaround implementation for both Tx csum errata.

Signed-off-by: Claudiu Manoil <claudiu.manoil@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>