NVMe: Fix divide-by-zero in nvme_trans_io_get_num_cmds

dev->max_hw_sectors may be zero to indicate the device has no limit on
the number of sectors.  nvme_trans_do_nvme_io() should use the software
limit, since this is guaranteed to be non-zero.

Reported-by: Mundu <mundu2510@gmail.com>
Signed-off-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Matthew Wilcox <matthew.r.wilcox@intel.com>