[TG3]: Refine DMA boundary setting.

Extract DMA boundary bit selection into a seperate
function, tg3_calc_dma_bndry().  Call this from
tg3_test_dma().

Make DMA test more reliable by using no DMA boundry
setting during the test.  If the test passes, then
use the setting we selected before the test.

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