usb: rmnet_sdio: Fix various crashes during composition switch

This changes fixes following stability issues:-
-Freeing of usb request twice:
USB requests are allocated in set_alt and freed in disable routine.
Therefore, do not free these again from unbind which gets called
during composition switch, and leads to memory corruption.

-Cancel delayed work before destroying workqueue:
This prevents system crash when delayed work is later queued to the
already destroyed workqueue.

-Attempt to close SDIO channels only if these are already open.
-Use unique names for debugfs files used by various rmnet drivers.

Change-Id: I3107f879a12c7b0292365778b8f6f191d409c651
Signed-off-by: Manu Gautam <mgautam@codeaurora.org>