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>
diff --git a/drivers/net/sfc/workarounds.h b/drivers/net/sfc/workarounds.h
index c821c15..3250299 100644
--- a/drivers/net/sfc/workarounds.h
+++ b/drivers/net/sfc/workarounds.h
@@ -41,6 +41,8 @@
 
 /* Spurious parity errors in TSORT buffers */
 #define EFX_WORKAROUND_5129 EFX_WORKAROUND_FALCON_A
+/* Unaligned read request >512 bytes after aligning may break TSORT */
+#define EFX_WORKAROUND_5391 EFX_WORKAROUND_FALCON_A
 /* iSCSI parsing errors */
 #define EFX_WORKAROUND_5583 EFX_WORKAROUND_FALCON_A
 /* RX events go missing */