net: dsa: mt7530: fix FDB entries not aging out with short timeout

net: dsa: mt7530: fix FDB entries not aging out with short timeout

The DSA forwarding selftests bridge_vlan_aware.sh and
bridge_vlan_unaware.sh configure the bridge with ageing_time set to
LOW_AGEING_TIME (1000 centiseconds, i.e. 10 seconds) and then run
learning_test() in lib.sh, which expects a learned FDB entry to be
removed after ageing_time + 10 seconds. On MT7530/MT7531 the entry
persisted past the deadline and the "Found FDB record when should
not" assertion failed.

With msecs=10000, the algorithm in mt7530_set_ageing_time() finds
AGE_CNT=0 and AGE_UNIT=9 as the first exact match (starting the
search from tmp_age_count=0). The per-entry aging counter is
initialized to AGE_CNT when a MAC address is learned, so with
AGE_CNT=0 new entries start with a counter value of 0, which the
hardware treats as "already aged" and never removes, effectively
disabling aging.

Fix this by starting the search from tmp_age_count=1 to ensure
entries always have a non-zero initial aging counter. For a
10-second ageing time this yields AGE_CNT=1 and AGE_UNIT=4 instead:
the timer ticks every 5 seconds and entries are removed after 2
ticks.

Starting the search at AGE_CNT=1 raises the minimum representable
ageing time from 1 to 2 seconds. Without bounds, a stale ageing_time
of 1 second would now make the loop fall through without setting
age_count and age_unit, leaving them uninitialized when written to
the MT7530_AAC hardware register. Set ds->ageing_time_min and
ds->ageing_time_max so the DSA core validates the range before the
callback is invoked, and drop the now-redundant range check from
mt7530_set_ageing_time().

Fixes: ea6d5c924e39 ("net: dsa: mt7530: support setting ageing time")
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Link: https://patch.msgid.link/7788ded12dc07b1bce329ec35fa70f4b45f3f9b7.1778766629.git.daniel@makrotopia.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>