usb: qc_ether: enable network interface only when requested

some protocols do not want to enable network interface untill control
plane is initialized, for example, rndis driver enables network
interface only after initialize/packet-filter control messages and
enabling network interface right after set_alt may result in race
conditions w/ dependent clients (like qti module).  Add flag to
enable network interface only when requested by ecm or rndis modules.

Change-Id: Ief410e3dfb685a8fc6206b8a58df94f200544ce3
Signed-off-by: Vamsi Krishna <vskrishn@codeaurora.org>