If scsi_host_template->host_reset is NULL and the user requests an
adapter reset through

	echo adapter > /sys/class/scsi_host/hostx/host_reset

-EINVAL will be returned even though the "adapter" argument is perfectly
valid.

Change this so that we only return -EINVAL if the provided string is
invalid.  If the host does not implement a ->host_reset function we'll
return -EOPNOTSUPP.

[mkp: tweaked patch description]
Signed-off-by: weiping zhang <zhangweiping@didichuxing.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>