i40e: Change BUG_ON to WARN_ON in service event complete

There's no need to kill the thread and eventually the kernel in this
case.  In fact, the remainder of the code won't hurt anything anyway,
so just complain that we're here and move along.

Prompted by a recent Linus diatribe.

Change-ID: Iec020d8bcfedffc1cd2553cc6905fd915bb3e670
Signed-off-by: Shannon Nelson <shannon.nelson@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>