can: esd_usb2: use DEVICE_ATTR_RO() helper macro

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

Link: https://lore.kernel.org/r/20210603110902.11930-1-thunder.leizhen@huawei.com


Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>