drivers: net: hamradio: Fix suspicious RCU usage warning in bpqether.c

This patch fixes the following warning:
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WARNING: suspicious RCU usage
5.7.0-rc5-next-20200514-syzkaller #0 Not tainted
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drivers/net/hamradio/bpqether.c:149 RCU-list traversed in non-reader section!!

Since rtnl lock is held, pass this cond in list_for_each_entry_rcu().

Reported-by:  <syzbot+bb82cafc737c002d11ca@syzkaller.appspotmail.com>
Signed-off-by: Madhuparna Bhowmik <madhuparnabhowmik10@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>