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.

Sponsored by: The FreeBSD Foundation
Reviewed by: hselasky
Differential Revision: https://reviews.freebsd.org/D29364

(cherry picked from commit a29bbfe6c6e41691bdd3bd0e5be89089523485d6)