PoE HAT driver cleanup

* Fix undeclared variable in rpi_poe_fan_suspend
* Add SPDX-License-Identifier
* Expand PoE acronym in Kconfig help
* Give clearer error message on of_property_count_u32_elems fail
* Add documentation
* Add vendor to of_device_id compatible string.
* Rename m_data_s struct to fw_data_s
* Fix typos

Fixes: #2665

Signed-off-by: Serge Schneider <serge@raspberrypi.org>