input: serio: use DEVICE_ATTR_RO()

Convert the serio sysfs fiels to use the DEVICE_ATTR_RO() macros to make
it easier to audit the correct sysfs file permission usage.

Cc: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Cc: <linux-input@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>