net: mscc: ocelot: fix fields offset in SG_CONFIG_REG_3

INIT_IPS and GATE_ENABLE fields have a wrong offset in SG_CONFIG_REG_3.
This register is used by stream gate control of PSFP, and it has not
been used before, because PSFP is not implemented in ocelot driver.

Signed-off-by: Xiaoliang Yang <xiaoliang.yang_1@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>