bridge: fix netconsole setup over bridge

Commit 93d8bf9f ("bridge: cleanup netpoll code") introduced
a check in br_netpoll_enable(), but this check is incorrect for
br_netpoll_setup(). This patch moves the code after the check
into __br_netpoll_enable() and calls it in br_netpoll_setup().
For br_add_if(), the check is still needed.

Fixes: 93d8bf9f

 ("bridge: cleanup netpoll code")
Cc: Toshiaki Makita <makita.toshiaki@lab.ntt.co.jp>
Cc: Stephen Hemminger <stephen@networkplumber.org>
Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: Cong Wang <cwang@twopensource.com>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Acked-by: Toshiaki Makita <makita.toshiaki@lab.ntt.co.jp>
Tested-by: Toshiaki Makita <makita.toshiaki@lab.ntt.co.jp>
Signed-off-by: David S. Miller <davem@davemloft.net>