netxen: handle pci bar 0 mapping failure

PCI bar 0 is used for memory mapped register access.
If ioremap fails (returns NULL), register access results
in crash.

Use pci_ioremap_bar() instead of ioremap(), the latter
fails on on 32 bit powerpc where pci resource address is
> 32 bits.

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