net: mrp: move struct definitions out of uapi

None of these are actually used in the kernel/userspace interface -
there's a userspace component of implementing MRP, and userspace will
need to construct certain frames to put on the wire, but there's no
reason the kernel should provide the relevant definitions in a UAPI
header.

In fact, some of those definitions were broken until previous commit,
so only keep the few that are actually referenced in the kernel code,
and move them to the br_private_mrp.h header.

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>