tg3: use dma_alloc_coherent() instead of pci_alloc_consistent()

Using dma_alloc_coherent() permits to use GFP_KERNEL allocations instead
of GFP_ATOMIC ones. Its better when a machine is out of memory, because
this allows driver to sleep to get its memory and succeed its init,
especially when allocating high order pages.

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