igb_uio: handle no IRQ fallback

igb_uio: handle no IRQ fallback

Fix a couple of issues with my earlier igb_uio stuff:
1. With MSI (like MSI-X) actual IRQ number is not known until
   after the pci_enable_msi() is done.
2. If INTX fails, fall back to running without IRQ.
   This allows usermode PCI to recover and run without out IRQ
   for cases where PCI INTX support is broken (aka VMWare).

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Acked-by: Thomas Monjalon <thomas.monjalon@6wind.com>