Previously, NMSettingInfiniband:verify() silently modifies the
setting for invalid MTU. verify() should not do that.

For libnm-core we can change behavior and implement normalization
of MTU. This changes behavior for NMSettingInfiniband:verify() so
that MTU gets no longer fixed by verify() alone. Instead verify()
fails with a verification error.

Due the possibility to normalize the MTU, NM still can receive
invalid settings and fix it.

For libnm-core we don't change behavior, merely add a code comment.

Signed-off-by: Thomas Haller <thaller@redhat.com>