net: usb: Block open call on QMI channels when device not connected

RMNET control driver returns error if modem device is not connected
and blocks otherwise when open gets called on QMI channels. QMUX daemon
bails out if open returns error during boot up and calls open later
on-demand as a request from its clients. This increases the overall
time to establish a data call.

Change-Id: I1ef00256893812c36c7596bf75ff5f9745bf61cd
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>