net: hsr: allow PRP RedBox (interlink) creation

With the PRP interlink datapath, duplicate discard and supervision support
in place, a PRP device can act as a RedBox. Remove the rtnetlink rejection
of "type hsr ... interlink <dev> proto 1"; the feature is implemented
unconditionally by the preceding patches.

Signed-off-by: Xin Xie <xiexinet@gmail.com>
Link: https://patch.msgid.link/20260717201457.54-4-xiexinet@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>