USB: ks_bridge: Prevent freeing buffers while copy is in progress

The completed read buffers are added to to_ks_list in Rx URB
completion handler.  The buffer is deleted from this list by
reader process after copying to user space buffer.  If disconnect
happens during this copy, the buffer is freed from disconnect
context.  Hence detach the buffer from to_ks_list before copying
to user space buffer.

CRs-Fixed: 448142
Change-Id: Ia4484b2709065162b0d32782a30e2b2fd36f5fda
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>