net: ethernet: ravb: constify bb_ops

The only usage of bb_ops is to assign its address to the ops field in
the mdiobb_ctrl struct, which is a const pointer. Make it const to allow
the compiler to put it in read-only memory.

Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>