net: ravb: Move DBAT configuration to the driver's ndo_open API

DBAT setup was done in the driver's probe API. As some IP variants switch
to reset mode (and thus registers content is lost) when setting clocks
(due to module standby functionality) to be able to implement runtime PM
move the DBAT configuration in the driver's ndo_open API.

This commit prepares the code for the addition of runtime PM.

Signed-off-by: Claudiu Beznea <claudiu.beznea.uj@bp.renesas.com>
Reviewed-by: Sergey Shtylyov <s.shtylyov@omp.ru>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>