ramips: mt7621: mikrotik 760igs (hEX S) fix SFP

ramips: mt7621: mikrotik 760igs (hEX S) fix SFP

This device uses an AR8031/AR8033 chip to convert SoC gmac1
RGMII to 1000base-x or sgmii for the SFP fibre cage.
The SFP cage requires phy-mode rgmii-rxid, and without it will not
recieve any packets: ethtool -S sfp rx_fcs_errors will increase when
packets should be being received, but no other _rx counters will change.

Fixes: c77858aa792 ("ramips: mt7621-dts: change phy-mode of gmac1 to rgmii")
Reviewed-by: Robert Marko <robimarko@gmail.com>
Reviewed-by: Arınç ÜNAL <arinc.unal@arinc9.com>
Signed-off-by: John Thomson <git@johnthomson.fastmail.com.au>
(cherry picked from commit 7ea965b57803ceec20222279377e611652ac217f)