usb: u_data_bridge: Lower DTR on USB cable disconnect to modem

When DUN call is operational and USB cable is being disconnected,
DTR is not being lowered with modem. This results into no DUN call
functionality on next USB cable connect case. Fix this issue by
lowering DTR to modem on USB cable disconnect.

This change also returns zero instead of -EAGAIN when there is no
write USB request available. Also it moves port open interrupted
message loglevel from pr_err() to pr_debug to reduce logging on
serial console.

Change-Id: I3259417aceee2713d7765c9de34ace4ea2d3ed96
Signed-off-by: Mayank Rana <mrana@codeaurora.org>