tg3: Simplify tx bd assignments

In the following patches, the process the driver will use to assign skb
fragments to transmit BDs will get more complicated.  To prepare for
that new code, this patch seeks to simplify how transmit BDs are
populated.  It does this by separating the code that assigns the BD
members from the logic that controls how the fields are set.

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