realtek: move port filtering out of uci_set_poe()

realtek: move port filtering out of uci_set_poe()

uci_set_poe() now performs two duties: filtering the list of device
ports to exclude non-PoE ports, and generating the PoE related device
config.

Extract the port filtering to an external function, which is made a bit
more readable by the use of 'sort -V [-r] | uniq -u' to filter duplicate
entries out of a (reverse) version sorted list.

Signed-off-by: Sander Vanheule <sander@svanheule.net>