nvmet: remove unreachable code

Get rid of the unreachable code in the nvmet_parse_discovery_cmd().
Keep the error message identical to the admin-cmd.c and io-cmd*.c

Signed-off-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>