Convert to use ->read_avail() instead of open-coded attribute handling.

While here, fix the typo in array definition and append comma in case of
the future extension.
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20200831090813.78841-2-andriy.shevchenko@linux.intel.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>