ofed/linuxkpi: use proper accessor function

In the notifier event callback function rather than casting directly
to the expected type use the proper accessor function as the mlx drivers
already do.
This is preparational work to allow us to improve the struct net_device
is struct ifnet compat code shortcut in the future.

Obtained-from: bz_iwlwifi
Sponsored-by: The FreeBSD Foundation
MFC-after: 2 weeks
Reviewed-by: hselasky
Differential Revision: https://reviews.freebsd.org/D29364