realtek: set power management mode

realtek: set power management mode

Discovered that EnGenius EWS2910P v1 appears to require a Set Power
Management Mode command for the port control to function.  Ensure
that the power management mode is set to dynamic power management
mode with priority. This should be safe for all gen1 and gen2 boards,
issue a warning if an error is detected.

Signed-off-by: Russell Senior <russell@personaltelco.net>
Link: https://github.com/openwrt/openwrt/pull/24550
Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>