eal/linux: fix irq handling with igb_uio

eal/linux: fix irq handling with igb_uio

The introduction of uio_pci_generic broke interrupt handling with
igb_uio. The igb_uio device uses the kernel read/write method to
enable disable IRQ's; the uio_pci_generic has to use PCI intx
config read/write to enable disable interrupts.

Since igb_uio uses MSI-X the PCI intx config read/write won't
work.

Fixes: c112df6875a5 ("eal/linux: toggle interrupt for uio_pci_generic")

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>