net: usb: Return error to QMI channels during mdm9x15 disconnect

Reads performed on rmnet control devices get blocked on wait queue.
Currently, reads get unblocked only when read response is received
from mdm9x15. Hence during mdm9x15 disconnect unblock reads to return
error to QMI channels.

Change-Id: I4a2cebe9d1ce45577da8cf8524f05178ebec47dd
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>