net: dsa: felix: keep QSYS_TAG_CONFIG_INIT_GATE_STATE(0xFF) out of rmw

In vsc9959_tas_clock_adjust(), the INIT_GATE_STATE field is not changed,
only the ENABLE field. Similarly for the disabling of the time-aware
shaper in vsc9959_qos_port_tas_set().

To reflect this, keep the QSYS_TAG_CONFIG_INIT_GATE_STATE_M mask out of
the read-modify-write procedure to make it clearer what is the intention
of the code.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>