i40e: inline interrupt enable

The interrupt enable function can be inlined by moving it to the header
file, which decreases the function call overhead for a frequently called
function.

Change-ID: I3214cc99593725768642680e7b8ce7e9bba7e44d
Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>