usb: ks_bridge: Free up stale buffers in probe

Driver can add data buffer to the link list of rx/tx
buffers after emptying the list during disconnect.
These stale buffers can confuse efs sync server when
mdm device connects back. Hence free up the rx/tx
buffers again during probe. Avoid further addition of
tx buffers after disconnect by checking device
connection status in write API. Also, move
misc_deregister() after cancelling the work in order
to remove device node early during disconnect to avoid
any write call further.

Change-Id: I0641669afcf1e1aec36d7c619dc8922495f446d0
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>