scsi: qedf: Use DEVICE_ATTR_RO() macro

Use DEVICE_ATTR_RO() macro helper instead of plain DEVICE_ATTR(), which
makes the code a bit shorter and easier to read.

Link: https://lore.kernel.org/r/20210616034419.725-3-thunder.leizhen@huawei.com
Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>