ramips: mt7620: fix RGMII TXID PHY mode

ramips: mt7620: fix RGMII TXID PHY mode

the register bits for TX delay and RX delay are opposites:
when TX delay bit is set, delay is enabled
when RX delay bit is set, delay is disabled

So, when both bits are unset, it is RX delay
and when both bits are set, it is TX delay

Note: TXID is the default RGMII mode of the SOC

Fixes: 5410a8e2959a ("ramips: mt7620: add rgmii delays support")
Signed-off-by: Michael Pratt <mcpratt@pm.me>
(cherry picked from commit 26c84b2e46caba1ae17bc82a533c99eee65e7004)