i40evf: invite vector 0 to the interrupt party

The i40evf_irq_enable and i40evf_fire_sw_interrupt functions were
unfairly discriminating against MSI-X vector 0, just because it doesn't
handle traffic. That doesn't mean it's not essential to the operation of
the driver. This change allows the watchdog to fire vector 0 via
software, which makes the driver tolerant of dropped interrupts on that
vector.

Buck up, vector 0! You can be part of our gang!

Change-ID: I37131d955018a6b3e711e1732d21428acd0d767e
Signed-off-by: Mitch Williams <mitch.a.williams@intel.com>
Tested-by: Jim Young <jamesx.m.young@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>