usb: gadget: qc_ecm: Remove setting of cdev and func to NULL

If host doesn't support remote wakeup and while performing
bus suspend functionality, currently cdev and func are being
set to NULL from ecm_qc_bam_disconnect(). Due to which resuming
device would never able to re-established USB IPA BAM
functionality. Fix this issue by removing setting of cdev and
func to NULL using data_inteface_up flag to check this condition
which would handle case where suspend is being received before
SET_ALT(1) is sent from host.

CRs-Fixed: 719802
Change-Id: I35076382916c3df720f256a2ac19c36c4da79407
Signed-off-by: Mayank Rana <mrana@codeaurora.org>