nvme-tcp: avoid open-coding nvme_tcp_teardown_admin_queue()

nvme_tcp_setup_ctrl() has an open-coded version of
nvme_tcp_teardown_admin_queue().

Signed-off-by: Hannes Reinecke <hare@suse.de>
Tested-by: Mark O'Donovan <shiftee@posteo.net>
Signed-off-by: Keith Busch <kbusch@kernel.org>