i40e: fixup legacy interrupt handling

There were a number of little bugs in the error handling of irq setup, most of
which ended up panicing the kernel, and are addressed by this patch, along with
a couple formatting issues.

Legacy interrupts (including MSI) are used only in the case of failure to
allocate MSI-X interrupts.

Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Tested-by: Kavindya Deegala <kavindya.s.deegala@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>