netxen: fix coherent dma mask setting

Change default dma mask for NX3031 to 39 bit with ability
to update it to 64-bit (if firmware indicates support). Old
code was restricting it under 4GB (32-bit), sometimes causing
failure to allocate descriptor rings on heavily populated
system. NX2031 based NICs will still get 32-bit coherent mask.

Signed-off-by: Dhananjay Phadke <dhananjay@netxen.com>
Signed-off-by: David S. Miller <davem@davemloft.net>