mlx5ib(4): Mark driver knows net epoch

This driver has already been EPOCH(9) aware since e48813009c6a.

Reviewed by: hselasky
Tested by: hselasky
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D39406

(cherry picked from commit da4068c4e1a090915a1a763c0d9fa5dc5b80842c)