net: sh_eth: Init the hardware before PHY access

To access the PHY, the MAC registers must be initialized. Call the init
function in probe() to make it so, otherwise the PHY ID readout returns
all zeroes.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Joe Hershberger <joe.hershberger@ni.com>
Cc: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>