net: miiphybb: Pass struct mii_dev directly to bb_miiphy_read/write()

net: miiphybb: Pass struct mii_dev directly to bb_miiphy_read/write()

Access to MDIO bus private data can be provided by both
struct mii_dev .priv member and struct bb_miiphy_bus .priv
member, use the former directly and remove .priv from the
later. Drop unused bb_miiphy_getbus(). This removes any
dependency on struct bb_miiphy_bus from the miiphybb code,
except for helper functions which will be removed later.

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