net/macb: fix capabilities configuration

Capabilities configuration by macb_configure_caps() was moved far too late by
421d9df0 (net/macb: merge at91_ether driver into macb driver) which would
lead to badly configured hardware.
So, move this function to early probe and modify its prototype to re-gain its
original behavior.
DT data retrieval is also moved to simplify the probe code flow.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Cc: Cyrille Pitchen <cyrille.pitchen@atmel.com>
Cc: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: David S. Miller <davem@davemloft.net>