net: usb: rmnet: Fix the issue between disconnect and completion handler

There is a issue between device disconnect and completion handler.
If the rmnet_usb_ctrl_disconnect function is happened before completion
handler. Then the system will be crashed after calling usb_autopm_put
_interface function since disconnect function frees rmnet ctrl device
memory and completion handler uses already freed memory. Adding check
for usb device ready state when called usb_autopm_put_interface function
to avoid crash.

CRs-fixed: 646272
Change-Id: I89ba1b899a29ccd82794f14599723681b817d483
Signed-off-by: Guang Jian Zeng <gzeng@codeaurora.org>