net: rmnet_usb: Block open until a response is received from MDM

commit 131eb4b6 (net: usb: Add MUX support in embedded rmnet driver)
has changed the behavior of port open.  Earlier the port open is
unblocked only after receiving notification available response on
the corresponding interface.  This commit unblocks the open immediately
after the corresponding interface probe is completed.  This is causing
functional failures at higher level.

Revert back to the old mechanism for non-muxed ports till the root
cause is identified.

CRs-Fixed: 469465
Change-Id: I2b53b3cac4d73f19ec4c9b1bc6f4ed18afbfabae
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>