par_master: Use new API to register shutdown function

This allows par masters to register shutdown function in par_master
struct, which means there is no need to call register_shutdown in init
function, since this call is now a part of register_par_master.

As a consequence of using new API, this patch also fixes propagation
of register_par_master() return values.

BUG=b:185191942
TEST=builds and ninja test

Change-Id: Ief7be907f53878b4b6567b52889735e5fff64ead
Signed-off-by: Anastasia Klimchuk <aklm@chromium.org>
---
M nic3com.c
M nicrealtek.c
2 files changed, 4 insertions(+), 15 deletions(-)