mv643xx_eth: get rid of compile-time configurable transmit checksumming

Get rid of the mv643xx_eth-internal MV643XX_ETH_CHECKSUM_OFFLOAD_TX
compile-time option.  Using transmit checksumming is the sane default,
and anyone wanting to disable it should use ethtool(8) instead of
recompiling their kernels.

Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>