realtek: pse: add patch for sysfs detection control

realtek: pse: add patch for sysfs detection control

Add a patch which modifies the Realtek PSE MCU driver backport to add a
sysfs control to control detection mode per port. Some older,
bad-designed devices aren't detected properly in standard mode on some
switches, though vendor firmware powers them properly. Testing shows
that enabling the detection of legacy PDs by allowing wider resistance,
more input capacitance and using a wider detection window, solves this
issue.

The legacy detection can be activated by writing 1 to
portN/detection_legacy in the device's sysfs, e.g.

  echo 1 > /sys/bus/i2c/devices/3-0020/port0/detection_legacy

This patch is kept downstream for now, upstreaming will be attempted
though it is likely that upstream prefers a generic pse-pd approach.

Link: https://github.com/openwrt/openwrt/pull/23222
Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>