ucc_geth: Don't use RX clock as TX clock.

Commit 9fb1e350

,
ucc_geth: use rx-clock-name and tx-clock-name device tree properties
Introduced a typo that made the driver use the RX clock
as TX clock, causing massive TX errors.

Signed-off-by: Joakim Tjernlund <Joakim.Tjernlund@transmode.se>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>