i40e: better return values

As mentioned by Joe Perches, clean up return values in some functions
making sure to have consistent return types, not mixing types.

A couple of Joe's comments suggested returning void, but since
the functions in question are ndo defined, the return values are fixed.
So make a comment in the header that notes this is a function called by
net_device_ops.

v2: fix post increment bug in return

CC: Joe Perches <joe@perches.com>
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>