net: ravb: Initialize PHY in probe() once

Reset and initialize the PHY once in the probe() function rather than
doing it over and over again is start() function. This requires us to
keep the clock enabled while the driver is in use. This significantly
reduces the time between transfers as the PHY doesn't have to restart
autonegotiation between transfers, which takes forever.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Geert Uytterhoeven <geert+renesas@glider.be>
Cc: Joe Hershberger <joe.hershberger@ni.com>