tg3: Consolidate code that calls tg3_tx_set_bd()

This patch consolidates all code that populates tx BDs into a single
routine.  Setting tx BDs needs to be more carefully controlled to see if
workarounds need to be applied.

Signed-off-by: Matt Carlson <mcarlson@broadcom.com>
Reviewed-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>