cdc_ether: fix autosuspend for mbm devices

Autosuspend works until you bring the wwan interface up, then the
device does not enter autosuspend anymore.

The following patch fixes the problem by setting the .manage_power
field in the mbm_info struct to the same as in the cdc_info struct
(cdc_manager_power).

Signed-off-by: Torgny Johansson <torgny.johansson@ericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>