generic: ar8216: update version switch for of_get_phy_mode fix

Kernel has changed the of_get_phy_mode API in commit 0c65b2b90d13
("net: of_get_phy_mode: Change API to solve int/unit warnings").

This is already included in kernel 5.5, so fix the version switch
(though this will not actually matter for the versions we support).

Similar driver adjustments to account for the API change will
probably be necessary to various other local drivers.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>