realtek: dsa: Add MSTI to HW MST ID mapping

realtek: dsa: Add MSTI to HW MST ID mapping

The MSTI range is 0..4095 but the HW range is only supporting a lower
range - for example 0..63 for RTL930x. But the HW doesn't really need to
know the actual MSTI. It is therefore possible to use a mapping from MSTI
to HW slot to allow a larger range of MSTIs.

Since the CIST (MSTI 0) is always needed, the mapping data structure is
skipping this entry and is always keeping the HW slot 0 for CIST.

This doesn't increase the total number of MSTIs a HW supports.

Suggested-by: Jonas Gorski <jonas.gorski@gmail.com>
Signed-off-by: Sven Eckelmann <se@simonwunderlich.de>
Link: https://github.com/openwrt/openwrt/pull/20421
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>