net: phy: lan87xx: Allow more time for link detect

With EDPWRDOWN set in idle, it must be cleared before checking for
ENERGYON going high, indicating that a link is being established.
The existing code allows 640ms for ENERGYON to go high, but on
Raspberry Pis that appears not to be enough, causing link detection
to fail.

Increase the polling timeout to 1500ms - with a polling interval of
10ms it shouldn't cause unnecessary delays.

See: https://github.com/raspberrypi/linux/issues/4393



Signed-off-by: Phil Elwell <phil@raspberrypi.com>