nvme-pci: use unsigned for io queue depth

The NVMe PCIe declares module parameter io_queue_depth as int. Change
this to u16 as queue depth can never be negative. Now to reflect this
update module parameter getter function from param_get_int() ->
param_get_uint() and respective setter function with type of n changed
from int to u16 with param_set_int() to param_set_ushort(). Finally
update struct nvme_dev q_depth member to u16 and use u16 in min_t()
when calculating dev->q_depth in the nvme_pci_enable() (since q_depth is
now u16) and use unsigned int instead of int when calculating
dev->tagset.queue_depth as target variable tagset->queue_depth is of type
unsigned int in nvme_dev_add().

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