nvme-core: use u16 type for directives

In nvme_configure_directives() when calculating number of streams use
u16 instead of unsigned type in the min_t() since target variable
ctrl->nr_streams is of type u16.

Signed-off-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>