mlx4: remove no longer needed header

Remove linux/inetdevice.h as neither of the two inline functions there
are used here.

Sposored-by: The FreeBSD Foundation
MFC-after: 2 weeks
Reviewed-by: hselasky
X-D-R: D29366 (extracted as further cleanup)
Differential Revision: https://reviews.freebsd.org/D29428