u_bam_data: Fix fast USB PO/PI case with ECM/RNDIS functionality

u_bam_data driver is one who provides USB cable disconnect and connect
notification to tethered function's IPA driver. With very fast USB PO/PI
case, it is seen that current way of using is_ipa_usb_connected flag is
not being updated although USB IPA driver is being connected. Due to this
on USB cable disconnect, USB IPA driver is not being teared down. Rename
is_ipa_usb_connected flag to is_net_interface_up to convey what is being
used for. Fix this issue by using this flag from bam_data_start_rx_tx()
API instead of bam2bam_data_connect_work() API for ECM/RNDIS. For MBIM,
it is not required to consider this flag as teth_bridge_init() is being
called in atomic context, teth_bridge_connect() is NO OPS API.

CRs-Fixed: 729725
Change-Id: I1ed6955ce7157fe120b98bde64c6931854b2dfeb
Signed-off-by: Mayank Rana <mrana@codeaurora.org>