usb: gadget: qc_ecm: add option to trigger Bulk eps from IPA

Provide a callback function that will be called from ipa side when
network interface is up ("ifconfig ecm0 up"). Upon getting this
notification the ECM USB function driver will initiate the bulk endpoints
queue. this way we ensure host will send DHCP discover only when device
side is ready.

Change-Id: Iccb4afdbb9cb7e3f68f16c1cce66254db1509048
Signed-off-by: Nadine Toledano <nadinet@codeaurora.org>
Signed-off-by: Lena Salman <esalman@codeaurora.org>