nvmet: remove local variable

In function errno_to_nvme_status() we store the value of the NVMe
status into the local variable and don't do anything useful with that
but just return.

Remove the local variable and return the value directly from switch.
This also removed extra break statements.

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