sfc: Move all TX DMA length limiting into tx.c

Replace the duplicated logic in efx_enqueue_skb() and
efx_tx_queue_insert() with an inline function, efx_max_tx_len().

Remove the failed attempt at abstracting hardware-specifics and put
all the magic numbers in efx_max_tx_len().

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>