staging: line6: use default sysfs attribute macros

Move to use the DEVICE_ATTR_RW/RO macros, making it easier to audit the
sysfs file permissions, and get rid of the "empty" callback for
read-only files, saving lines of code.

Cc: Stefan Hajnoczi <stefanha@gmail.com>
Cc: Laurent Navet <laurent.navet@gmail.com>
Cc: Johannes Thumshirn <morbidrsa@googlemail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>