tg3: Set tx bug flags for more devices

It has been recently discovered that all tg3 devices have a 4Gb boundary
DMA problem, and that all 5755 and newer devices can't handle fragments
less than or equal to 8 bytes in size.  This patch adjusts the flags and
removes tg3_start_xmit().  tg3_start_xmit_dma_bug() has been renamed to
tg3_start_xmit().

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