x86/microcode: Use the DEVICE_ATTR_RO() macro

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

No change in functionality.

Signed-off-by: Guangju Wang[baidu] <wgj900@163.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20230118023554.1898-1-wgj900@163.com