libata: fix ata_host_start()

libata: fix ata_host_start()

The loop on entry of ata_host_start() may not initialize host->ops to a
non NULL value. The test on the host_stop field of host->ops must then
be preceded by a check that host->ops is not NULL.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Link: https://lore.kernel.org/r/20210816014456.2191776-3-damien.lemoal@wdc.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>