diag: Wake up diag_socket_read() once the diag channel is closed

Wake up the diag_socket_read() function immediately after setting
the diag channel handle to NULL so that diag_socket_read() can
complete its execution when cancel_work_sync() is called in close
call.

Change-Id: I6ac2b3301a3ae781b69a2ca9c7c518576dd22fbb
Signed-off-by: Sreelakshmi Gownipalli <sgownipa@codeaurora.org>
Signed-off-by: Bruce Levy <blevy@codeaurora.org>