RDMA/srp: Use DEVICE_ATTR_*() macros

Use DEVICE_ATTR_*() helpers instead of plain DEVICE_ATTR, which makes the
code a bit shorter and easier to read.

Link: https://lore.kernel.org/r/20210528125750.20788-1-yuehaibing@huawei.com


Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>