realtek: EnGenius EWS2910P: add support for SFP ports

realtek: EnGenius EWS2910P: add support for SFP ports

The SFP cages 9F and 10F share the same SCL line. Currently, there
isn't a good way to model this. Thus, only one SFP port can be fully
supported.

Cage 10F is fully supported with an I2C bus and sfp handle. Linux
automatically handles enabling or disabling the TX laser.

Cage 9F is only parially supported, without the sfp handle. The SDA
line is hogged as an input, so that it remains high. SCL transitions
sould not affect modules connected to this cage. The default value of
the tx-disable line is high (active). It is exported as a gpio, but
the laser is off by default. To enable the laser:

    echo 0 > /sys/class/gpio/sff-p9-tx-disable/value

Thus, both modules can be used for networking, but only 10F will be
able to detect and identify a plugged in SFP module.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>