phy: Fix node description of phy_get_by_node

node is most of device related API's are termed as device
node and without device related API's are termed as ofnode.

generic_phy_get_by_node API is without device API, so fixed
the node description as ofnode.

Cc: Neil Armstrong <narmstrong@baylibre.com>
Cc: Tom Rini <trini@konsulko.com>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>