net: enetc: save the mode register address inside struct enetc_cbdr

enetc_clear_cbdr depends on struct enetc_hw because it must disable the
ring through a register write. We'd like to remove that dependency, so
let's do what's already done with the producer and consumer indices,
which is to save the iomem address in a variable kept in struct enetc_cbdr.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>