media: ddbridge: remove unused MDIO defines and hwinfo member

ddbridge has a few MDIO related remainders (defines, hwinfo struct) which
aren't of any use for the in-kernel driver at all (they're only used in
conjunction with the OctoNet SAT>IP boxes which the kernel driver doesn't
have any support for), so clean this up.

Signed-off-by: Daniel Scheller <d.scheller@gmx.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>