net: ravb: Drop empty init callback

net: ravb: Drop empty init callback

The init function does nothing, the bb_miiphy_init() already checks
whether the .init callback is assigned, and if not, skips calling it.
Remove the empty init function. The entire init callback will be
removed in follow up patches.

Reviewed-by: Paul Barker <paul.barker.ct@bp.renesas.com>
Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>