netxen: fix msi-x interrupt handling

o Cut down msi-x vectors from 8 to 1 since only one is used for now.
o Use separate handler for msi-x, that doesn't unnecessarily scrub
  msi status register.

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