drivers: shsusr: return a disconnect return code when shs is cleaning up

Returns a return code indicating rmnet_shs is cleaning up. This is so shsusrd
is notified that the cleanup is about to happen.

Change-Id: Ia11b85426a446ca9a599cbe0fc2117a996d67c37
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>