ath79: mikrotik: enable SFP on RB922UAGS-5HPaCD

ath79: mikrotik: enable SFP on RB922UAGS-5HPaCD

This patch enables the SFP cage on the MikroTik RouterBOARD 922UAGS-5HPacD.

GPIO16 (tx-disable-gpios) should be governed by the SFP driver to enable
or disable transmission, but no change is observed. Therefore, it is
left as output high to ensure the SFP module is forced to transmit.

Tested on a RouterBOARD 922UAGS-5HPacD board, with a CISCO GLC-LH-SMD
1310nm module and an unbranded GLC-T RJ45 Gigabit module. PC=>router
iperf3 tests deliver 440/300 Mbps up/down, both via regular eth0 port
or SFP port with RJ45 module. Bridge between eth0 and eth1 delivers
950 Mbps symmetric.

Signed-off-by: Roger Pueyo Centelles <roger.pueyo@guifi.net>