nvme: reduce checks for zero command effects

For passthrough I/O commands, effects are usually to be zero.
nvme_passthrough_end() does three checks in futility for this case.
Bail out of function-call/checks.

Signed-off-by: Kanchan Joshi <joshi.k@samsung.com>
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>