ath79: fix gigabit link pll-data for EX7300

ath79: fix gigabit link pll-data for EX7300

The device did not appear to be reachable unless the connection were
forced to 100Mb or lower. Revert to previously working pll-data.

Also fix the phy-mode to represent the actual state needed for ethernet
to function.

Reported-by: Moritz Schreiber <moritz@mosos.de>
Signed-off-by: Daniel Gimpelevich <daniel@gimpelevich.san-francisco.ca.us>
[add remark about phy-mode property]
Signed-off-by: David Bauer <mail@david-bauer.net>
(cherry picked from commit ee41b602a2a2ced06c26e6edc160b2a5e9619f0d)