caif: __dev_get_by_index instead of dev_get_by_index to find interface

The following call chains indicate that chnl_net_open() is under
rtnl_lock protection as __dev_open() is protected by rtnl_lock.
So if __dev_get_by_index() instead of dev_get_by_index() is used
to find interface handler in it, this would help us avoid to change
interface reference counter.

__dev_open()
  chnl_net_open()

Cc: Dmitry Tarnyagin <dmitry.tarnyagin@lockless.no>
Signed-off-by: Ying Xue <ying.xue@windriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>