scsi: megaraid_mbox: Use DEVICE_ATTR_ADMIN_RO() macro

Use DEVICE_ATTR_ADMIN_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/20210616034419.725-4-thunder.leizhen@huawei.com
Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>