Revert r291170

The mlx5* driver(s) are built [*]/installed separate from the OFED stack thanks
to recent refactoring done in the linuxkpi(4) module.

Always install the manpages instead of conditionally installing them if
MK_OFED != no

• Further refactoring of sys/ofed and linuxkpi(4) is pending to fully divorce mlx5* from ofed headers

MFC after: never
Requested by: hps