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 <[email protected]>
Signed-off-by: Daniel Gimpelevich <[email protected]>
[add remark about phy-mode property]
Signed-off-by: David Bauer <[email protected]>
(cherry picked from commit ee41b602a2a2ced06c26e6edc160b2a5e9619f0d)