iio: pressure: bmp280: Convert to use ->read_avail()

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

Suggested-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>