net: mscc: ocelot: fix address of SYS_COUNT_TX_AGING counter

This register, used as part of stats->tx_dropped in
ocelot_get_stats64(), has a wrong address. At the address currently
given, there is actually the c_tx_green_prio_6 counter.

Fixes: a556c76a ("net: mscc: Add initial Ocelot switch support")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>