atlx: fix warning in drivers/net/atlx/atl2.c

fix this warning:

  drivers/net/atlx/atl2.c: In function ‘atl2_request_irq’:
  drivers/net/atlx/atl2.c:644: warning: unused variable ‘err’

'err' is unused in the !CONFIG_PCI_MSI case.

Instead of further increasing the #ifdeffery in this function,
restructure the code a bit and get rid of the #ifdef. This
relies on the fact that pci_enable_msi() will always fail in
the !CONFIG_PCI_MSI case.

There should be no change in driver behavior.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: David S. Miller <davem@davemloft.net>